diff --git a/test/irb/test_debug_cmd.rb b/test/irb/test_debug_cmd.rb
index 0242baa26..3ba874230 100644
--- a/test/irb/test_debug_cmd.rb
+++ b/test/irb/test_debug_cmd.rb
@@ -198,6 +198,8 @@ def test_catch
 
     private
 
+    TIMEOUT_SEC = 3
+
     def run_ruby_file(&block)
       cmd = [EnvUtil.rubybin, "-I", LIB, @ruby_file.to_path]
       tmp_dir = Dir.mktmpdir
@@ -211,7 +213,7 @@ def run_ruby_file(&block)
       yield
 
       PTY.spawn(IRB_AND_DEBUGGER_OPTIONS.merge("IRBRC" => rc_file.to_path), *cmd) do |read, write, pid|
-        Timeout.timeout(3) do
+        Timeout.timeout(TIMEOUT_SEC) do
           while line = safe_gets(read)
             lines << line
 
@@ -226,6 +228,7 @@ def run_ruby_file(&block)
       ensure
         read.close
         write.close
+        kill_safely(pid)
       end
 
       lines.join
@@ -251,6 +254,35 @@ def safe_gets(read)
       nil
     end
 
+    def kill_safely pid
+      return if wait_pid pid, TIMEOUT_SEC
+
+      Process.kill :TERM, pid
+      return if wait_pid pid, 0.2
+
+      Process.kill :KILL, pid
+      Process.waitpid(pid)
+    rescue Errno::EPERM, Errno::ESRCH
+    end
+
+    def wait_pid pid, sec
+      total_sec = 0.0
+      wait_sec = 0.001 # 1ms
+
+      while total_sec < sec
+        if Process.waitpid(pid, Process::WNOHANG) == pid
+          return true
+        end
+        sleep wait_sec
+        total_sec += wait_sec
+        wait_sec *= 2
+      end
+
+      false
+    rescue Errno::ECHILD
+      true
+    end
+
     def type(command)
       @commands << command
     end