diff --git a/ChangeLog b/ChangeLog
index 200fffff82..e6244cbd46 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Jul 20 12:27:56 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/fileutils.rb (FileUtils::Entry_#copy): check file name
+	  spearator boundary.  [ruby-core:31360]
+
 Mon Jul 19 18:34:12 2010  Tanaka Akira  <akr@fsij.org>
 
 	* ext/pathname/pathname.c (get_strpath): check the type.
diff --git a/lib/fileutils.rb b/lib/fileutils.rb
index 5905022c96..73e2bfe3c3 100644
--- a/lib/fileutils.rb
+++ b/lib/fileutils.rb
@@ -1240,7 +1240,7 @@ module FileUtils
       when file?
         copy_file dest
       when directory?
-        if !File.exist?(dest) and /^#{Regexp.quote(path)}/ =~ File.dirname(dest)
+        if !File.exist?(dest) and descendant_diretory?(dest, path)
           raise ArgumentError, "cannot copy directory %s to itself %s" % [path, dest]
         end
         begin
@@ -1394,6 +1394,17 @@ module FileUtils
       return File.path(base) if not dir or dir == '.'
       File.join(dir, base)
     end
+
+    if File::ALT_SEPARATOR
+      DIRECTORY_TERM = "(?=[/#{Regexp.quote(File::ALT_SEPARATOR)}]|\\z)".freeze
+    else
+      DIRECTORY_TERM = "(?=/|\\z)".freeze
+    end
+    SYSCASE = File::FNM_SYSCASE.nonzero? ? "-i" : ""
+
+    def descendant_diretory?(descendant, ascendant)
+      /\A(?#{SYSCASE}:#{Regexp.quote(ascendant)})#{DIRECTORY_TERM}/ =~ File.dirname(descendant)
+    end
   end   # class Entry_
 
   def fu_list(arg)   #:nodoc:
diff --git a/test/fileutils/test_fileutils.rb b/test/fileutils/test_fileutils.rb
index f385589d0a..494204ced5 100644
--- a/test/fileutils/test_fileutils.rb
+++ b/test/fileutils/test_fileutils.rb
@@ -286,6 +286,15 @@ class TestFileUtils
     assert_directory 'tmp/cpr_dest/d'
     my_rm_rf 'tmp/cpr_src'
     my_rm_rf 'tmp/cpr_dest'
+
+    bug3588 = '[ruby-core:31360]'
+    assert_nothing_raised(ArgumentError, bug3588) do
+      cp_r 'tmp', 'tmp2'
+    end
+    assert_directory 'tmp2/tmp'
+    assert_raise(ArgumentError, bug3588) do
+      cp_r 'tmp2', 'tmp2/new_tmp2'
+    end
   end
 
   def test_cp_r_symlink