diff --git a/app/assets/stylesheets/atoms/_a-user-name.sass b/app/assets/stylesheets/atoms/_a-user-name.sass
index dbcdb8a8316..2e9fc209110 100644
--- a/app/assets/stylesheets/atoms/_a-user-name.sass
+++ b/app/assets/stylesheets/atoms/_a-user-name.sass
@@ -2,6 +2,7 @@
   color: $muted-text
   font-size: .8125rem
   line-height: 1.4
+  white-space: nowrap
 
 a.a-user-name
   +hover-link
diff --git a/app/assets/stylesheets/blocks/thread/_thread-list-item.sass b/app/assets/stylesheets/blocks/thread/_thread-list-item.sass
index fe731624ded..3ea3f3825de 100644
--- a/app/assets/stylesheets/blocks/thread/_thread-list-item.sass
+++ b/app/assets/stylesheets/blocks/thread/_thread-list-item.sass
@@ -37,19 +37,15 @@
     color: $reversal-text
     border-color: #b50f38
 
-
 .thread-list-item__inner
   +position(relative)
   padding-left: 3.75rem
   +media-breakpoint-down(sm)
     padding-left: 3rem
-  .thread-list-item.has-assigned &
-    +media-breakpoint-up(md)
-      padding-right: 8.75rem
 
 .thread-list-item__assignee
   +media-breakpoint-up(md)
-    +position(absolute, right 0, top 0)
+    +position(absolute, right -.5rem, top -.25rem)
     width: 8rem
   +media-breakpoint-down(sm)
     margin-top: .75rem
@@ -118,14 +114,19 @@
     opacity: .6
 
 .thread-list-item__row-separator
-  // TODO resetに移す
-  border: none
-  margin-bottom: .25rem
   border-top: dashed 1px $border
+  height: 1px
+  +media-breakpoint-up(md)
+    +margin(vertical, .5rem)
+  +media-breakpoint-down(sm)
+    +margin(vertical, .75rem .25rem)
 
 .thread-list-item__row
   &:not(:first-child)
     margin-top: .25rem
+  .thread-list-item.has-assigned &:first-child
+    +media-breakpoint-up(md)
+      padding-right: 8.5rem
 
 .thread-list-item__show-user-detail
   +position(absolute, top 0)
diff --git a/app/javascript/comments.vue b/app/javascript/comments.vue
index a2e4f828e1d..84dcaa61f22 100644
--- a/app/javascript/comments.vue
+++ b/app/javascript/comments.vue
@@ -1,7 +1,7 @@
 <template lang="pug">
-.thread-comments(v-if='loaded === false')
+#comments.thread-comments(v-if='loaded === false')
   commentPlaceholder(v-for='num in placeholderCount', :key='num')
-.thread-comments(v-else)
+#comments.thread-comments(v-else)
   comment(
     v-for='(comment, index) in comments',
     :key='comment.id',
diff --git a/app/javascript/product.vue b/app/javascript/product.vue
index 4fd97d3f3a1..d54dc5981cc 100644
--- a/app/javascript/product.vue
+++ b/app/javascript/product.vue
@@ -1,5 +1,5 @@
 <template lang="pug">
-.thread-list-item(:class='product.wip ? "is-wip" : ""')
+.thread-list-item.has-assigned(:class='product.wip ? "is-wip" : ""')
   .thread-list-item__strip-label(v-if='notResponded || unassigned')
     .thread-list-item__elapsed-days.is-reply-warning.is-only-mentor(
       v-if='isLatestProductSubmittedJust5days'
@@ -29,25 +29,26 @@
           .thread-list-item-meta__items
             .thread-list-item-meta__item
               a.a-user-name(:href='product.user.url') {{ product.user.login_name }}
+      .thread-list-item__row
+        .thread-list-item-meta
+          .thread-list-item-meta__items
             .thread-list-item-meta__item(v-if='product.wip')
               .a-meta 提出物作成中
             .thread-list-item-meta__item(v-else-if='product.published_at')
-              time.a-meta(datetime='product.published_at_date_time')
+              time.a-meta
                 span.a-meta__label 提出日
                 | {{ product.published_at }}
             .thread-list-item-meta__item(v-else)
-              time.a-meta(datetime='product.created_at_date_time')
+              time.a-meta
                 span.a-meta__label 提出日
                 | {{ product.created_at }}
-            time.a-meta(
-              v-if='product.updated_at',
-              datetime='product.updated_at_date_time'
-            )
-              span.a-meta__label 更新
-              | {{ product.updated_at }}
+            .thread-list-item-meta__item
+              time.a-meta(v-if='product.updated_at')
+                span.a-meta__label 更新
+                | {{ product.updated_at }}
 
+      hr.thread-list-item__row-separator(v-if='product.comments.size > 0')
       .thread-list-item__row(v-if='product.comments.size > 0')
-        hr.thread-list-item__row-separator
         .thread-list-item-meta
           .thread-list-item-meta__items
             .thread-list-item-meta__item
@@ -67,10 +68,33 @@
                       :src='user.avatar_url',
                       :class='[roleClass, daimyoClass]'
                     )
-                time.a-meta(
-                  datetime='product.comments.last_created_at_date_time'
-                )
-                  | 〜 {{ product.comments.last_created_at }}
+
+            .thread-list-item-meta__item(
+              v-if='product.self_last_comment_at_date_time && product.mentor_last_comment_at_date_time'
+            )
+              time.a-meta(
+                v-if='product.self_last_comment_at_date_time > product.mentor_last_comment_at_date_time'
+              )
+                | 〜 {{ product.self_last_comment_at }}（
+                strong
+                  | 提出者
+                | ）
+              time.a-meta(
+                v-if='product.self_last_comment_at_date_time < product.mentor_last_comment_at_date_time'
+              )
+                | 〜 {{ product.mentor_last_comment_at }}（メンター）
+
+            .thread-list-item-meta__item(
+              v-else-if='product.self_last_comment_at_date_time || product.mentor_last_comment_at_date_time'
+            )
+              time.a-meta(v-if='product.self_last_comment_at_date_time')
+                | 〜 {{ product.self_last_comment_at }}（
+                strong
+                  | 提出者
+                | ）
+              time.a-meta(v-else-if='product.mentor_last_comment_at_date_time')
+                | 〜 {{ product.mentor_last_comment_at }}（メンター）
+
     .stamp.stamp-approve(v-if='product.checks.size > 0')
       h2.stamp__content.is-title 確認済
       time.stamp__content.is-created-at {{ product.checks.last_created_at }}
diff --git a/app/javascript/report.vue b/app/javascript/report.vue
index 0245ad9b3d4..b729aa1304c 100644
--- a/app/javascript/report.vue
+++ b/app/javascript/report.vue
@@ -37,8 +37,8 @@
             .thread-list-item-meta__item
               time.a-meta {{ report.reportedOn }}
                 | の日報
+      hr.thread-list-item__row-separator
       .thread-list-item__row(v-if='report.hasAnyComments')
-        hr.thread-list-item__row-separator
         .thread-list-item-meta
           .thread-list-item-meta__items
             .thread-list-item-meta__item
diff --git a/app/models/comment.rb b/app/models/comment.rb
index 51318f596af..14fb3cc306d 100644
--- a/app/models/comment.rb
+++ b/app/models/comment.rb
@@ -8,6 +8,7 @@ class Comment < ApplicationRecord
   belongs_to :user, touch: true
   belongs_to :commentable, polymorphic: true
   after_create CommentCallbacks.new
+  after_update CommentCallbacks.new
   after_destroy CommentCallbacks.new
   alias sender user
 
diff --git a/app/models/comment_callbacks.rb b/app/models/comment_callbacks.rb
index 188914c63fe..6af984bffd4 100644
--- a/app/models/comment_callbacks.rb
+++ b/app/models/comment_callbacks.rb
@@ -11,17 +11,55 @@ def after_create(comment)
 
     return unless comment.commentable.instance_of?(Product)
 
+    update_last_comment_at(comment)
     delete_product_cache(comment.commentable.id)
   end
 
+  def after_update(comment)
+    return unless comment.commentable.instance_of?(Product)
+
+    update_last_comment_at(comment)
+  end
+
   def after_destroy(comment)
     return unless comment.commentable.instance_of?(Product)
 
+    delete_last_comment_at(comment.commentable.id)
     delete_product_cache(comment.commentable.id)
   end
 
   private
 
+  def reset_last_comment_at
+    product.mentor_last_comment_at = nil
+    product.self_last_comment_at = nil
+  end
+
+  def delete_last_comment_at(product_id)
+    product = Product.find(product_id)
+
+    reset_last_comment_at
+
+    product.comments.each do |comment|
+      if comment.user.mentor
+        product.mentor_last_comment_at = comment.updated_at
+      elsif comment.user == product.user
+        product.self_last_comment_at = comment.updated_at
+      end
+    end
+    product.save!
+  end
+
+  def update_last_comment_at(comment)
+    product = Product.find(comment.commentable.id)
+    if comment.user.mentor
+      product.mentor_last_comment_at = comment.updated_at
+    elsif comment.user == product.user
+      product.self_last_comment_at = comment.updated_at
+    end
+    product.save!
+  end
+
   def notify_comment(comment)
     NotificationFacade.came_comment(
       comment,
diff --git a/app/views/api/products/_product.json.jbuilder b/app/views/api/products/_product.json.jbuilder
index 2a8115f3372..8d8e551593f 100644
--- a/app/views/api/products/_product.json.jbuilder
+++ b/app/views/api/products/_product.json.jbuilder
@@ -17,6 +17,16 @@ if product.updated_at.present?
   json.updated_at_date_time product.updated_at.to_datetime
 end
 
+if product.self_last_comment_at.present?
+  json.self_last_comment_at l(product.self_last_comment_at)
+  json.self_last_comment_at_date_time product.self_last_comment_at.to_datetime
+end
+
+if product.mentor_last_comment_at.present?
+  json.mentor_last_comment_at l(product.mentor_last_comment_at)
+  json.mentor_last_comment_at_date_time product.mentor_last_comment_at.to_datetime
+end
+
 json.user do
   json.partial! "api/users/user", user: product.user
 end
diff --git a/app/views/products/_report.html.slim b/app/views/products/_report.html.slim
index 2480b364a15..b5e05d5c78c 100644
--- a/app/views/products/_report.html.slim
+++ b/app/views/products/_report.html.slim
@@ -20,8 +20,8 @@
                 = l report.reported_on
                 = 'の日報'
       - if report.comments.any?
+        hr.thread-list-item__row-separator
         .thread-list-item__row
-          hr.thread-list-item__row-separator
           .thread-list-item-meta
             .thread-list-item-meta__items
               .thread-list-item-meta__item
diff --git a/app/views/products/show.html.slim b/app/views/products/show.html.slim
index 75cba5e195b..e51785e2155 100644
--- a/app/views/products/show.html.slim
+++ b/app/views/products/show.html.slim
@@ -49,7 +49,26 @@ header.page-header
                     .thread-header-metas__meta
                       = link_to @product.user, class: 'a-user-name' do
                         = @product.user.long_name
-
+                  .thread-header-metas__end
+                    .thread-header-metas__meta
+                      - length = @product.comments.length
+                      a.a-meta(href='#comments' class="#{length.zero? ? 'is-disabled' : ''}")
+                        | コメント（
+                        span(class="#{length.zero? ? 'is-muted' : 'is-emphasized'}")
+                          = length
+                        | ）
+              .thread-header__row
+                h1.thread-header-title(class="#{@product.wip? ? 'is-wip' : ''}")
+                  - if @product.wip?
+                    span.thread-header-title__label
+                      | WIP
+                  span.thread-header-title__title
+                    - if @product.user.daimyo?
+                      | ★
+                    | #{link_to @product.practice.title, @product.practice, class: 'thread-header-title__link'}の提出物
+              .thread-header__row
+                .thread-header-metas
+                  .thread-header-metas__start
                     .thread-header-metas__meta
                       .a-meta
                         - if @product.wip?
@@ -74,24 +93,6 @@ header.page-header
                           time.a-meta__value(datetime="#{@product.updated_at.to_datetime}")
                             = l @product.updated_at
 
-                    .thread-header-metas__meta
-                      - length = @product.comments.length
-                      a.a-meta(href='#comments' class="#{length.zero? ? 'is-disabled' : ''}")
-                        | コメント（
-                        span(class="#{length.zero? ? 'is-muted' : 'is-emphasized'}")
-                          = length
-                        | ）
-
-              .thread-header__row
-                h1.thread-header-title(class="#{@product.wip? ? 'is-wip' : ''}")
-                  - if @product.wip?
-                    span.thread-header-title__label
-                      | WIP
-                  span.thread-header-title__title
-                    - if @product.user.daimyo?
-                      | ★
-                    | #{link_to @product.practice.title, @product.practice, class: 'thread-header-title__link'}の提出物
-
               .thread-header__row
                 .thread-header-actions
                   .thread-header-actions__start
diff --git a/app/views/reports/_report.html.slim b/app/views/reports/_report.html.slim
index 0068b67e52c..00bfc9563c3 100644
--- a/app/views/reports/_report.html.slim
+++ b/app/views/reports/_report.html.slim
@@ -39,8 +39,8 @@
                 = l report.reported_on
                 = 'の日報'
       - if report.comments.any?
+        hr.thread-list-item__row-separator
         .thread-list-item__row
-          hr.thread-list-item__row-separator
           .thread-list-item-meta
             .thread-list-item-meta__items
               .thread-list-item-meta__item
diff --git a/db/data/20210816082555_add_last_comment_at.rb b/db/data/20210816082555_add_last_comment_at.rb
new file mode 100644
index 00000000000..4133ae3d0b3
--- /dev/null
+++ b/db/data/20210816082555_add_last_comment_at.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class AddLastCommentAt < ActiveRecord::Migration[6.1]
+  def up
+    products = Product.where(self_last_comment_at: nil).where(mentor_last_comment_at: nil)
+    products.each do |product|
+      next unless product.comments.size.positive?
+
+      product.comments.each do |comment|
+        if comment.user.mentor
+          product.mentor_last_comment_at = comment.updated_at
+        elsif comment.user == product.user
+          product.self_last_comment_at = comment.updated_at
+        end
+      end
+      product.save!
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end
diff --git a/db/migrate/20210805061849_add_self_last_comment_at_column_and_mentor_last_comment_at_column_to_products.rb b/db/migrate/20210805061849_add_self_last_comment_at_column_and_mentor_last_comment_at_column_to_products.rb
new file mode 100644
index 00000000000..bb189150783
--- /dev/null
+++ b/db/migrate/20210805061849_add_self_last_comment_at_column_and_mentor_last_comment_at_column_to_products.rb
@@ -0,0 +1,7 @@
+class AddSelfLastCommentAtColumnAndMentorLastCommentAtColumnToProducts < ActiveRecord::Migration[6.1]
+  def change
+    add_column :products, :self_last_comment_at, :datetime
+
+    add_column :products, :mentor_last_comment_at, :datetime
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index c463cbf16b5..6cccf3b95bd 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -322,6 +322,8 @@
     t.boolean "wip", default: false, null: false
     t.datetime "published_at"
     t.bigint "checker_id"
+    t.datetime "self_last_comment_at"
+    t.datetime "mentor_last_comment_at"
     t.index ["practice_id"], name: "index_products_on_practice_id"
     t.index ["user_id", "practice_id"], name: "index_products_on_user_id_and_practice_id", unique: true
     t.index ["user_id"], name: "index_products_on_user_id"