Index: test/unit/lib/redmine/unified_diff_test.rb
===================================================================
--- test/unit/lib/redmine/unified_diff_test.rb	(revision 12041)
+++ test/unit/lib/redmine/unified_diff_test.rb	(working copy)
@@ -308,6 +308,54 @@
     end
   end
 
+  def test_offset_range_japanese_3
+    # UTF-8 The 1st byte differs.
+    ja1 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe8\xa8\x98</span>"
+    ja1.force_encoding('UTF-8') if ja1.respond_to?(:force_encoding)
+    ja2 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe5\xa8\x98</span>"
+    ja2.force_encoding('UTF-8') if ja2.respond_to?(:force_encoding)
+    with_settings :repositories_encodings => '' do
+      diff = Redmine::UnifiedDiff.new(
+               read_diff_fixture('issue-13644-3.diff'), :type => 'sbs')
+      assert_equal 1, diff.size
+      assert_equal 3, diff.first.size
+      assert_equal ja1, diff.first[1].html_line_left
+      assert_equal ja2, diff.first[1].html_line_right
+    end
+  end
+
+  def test_offset_range_japanese_4
+    # UTF-8 The 2nd byte differs. 
+    ja1 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe8\xa8\x98</span>"
+    ja1.force_encoding('UTF-8') if ja1.respond_to?(:force_encoding)
+    ja2 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe8\xaa\x98</span>"
+    ja2.force_encoding('UTF-8') if ja2.respond_to?(:force_encoding)
+    with_settings :repositories_encodings => '' do
+      diff = Redmine::UnifiedDiff.new(
+               read_diff_fixture('issue-13644-4.diff'), :type => 'sbs')
+      assert_equal 1, diff.size
+      assert_equal 3, diff.first.size
+      assert_equal ja1, diff.first[1].html_line_left
+      assert_equal ja2, diff.first[1].html_line_right
+    end
+  end
+
+  def test_offset_range_japanese_5
+    # UTF-8 The 2nd byte differs. 
+    ja1 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe8\xa8\x98</span>ok"
+    ja1.force_encoding('UTF-8') if ja1.respond_to?(:force_encoding)
+    ja2 = "\xe6\x97\xa5\xe6\x9c\xac<span>\xe8\xaa\x98</span>ok"
+    ja2.force_encoding('UTF-8') if ja2.respond_to?(:force_encoding)
+    with_settings :repositories_encodings => '' do
+      diff = Redmine::UnifiedDiff.new(
+               read_diff_fixture('issue-13644-5.diff'), :type => 'sbs')
+      assert_equal 1, diff.size
+      assert_equal 3, diff.first.size
+      assert_equal ja1, diff.first[1].html_line_left
+      assert_equal ja2, diff.first[1].html_line_right
+    end
+  end
+
   private
 
   def read_diff_fixture(filename)
Index: test/fixtures/diffs/issue-13644-4.diff
===================================================================
--- test/fixtures/diffs/issue-13644-4.diff	(revision 0)
+++ test/fixtures/diffs/issue-13644-4.diff	(revision 0)
@@ -0,0 +1,7 @@
+--- a.txt	2013-07-27 04:20:45.973229414 +0900
++++ b.txt	2013-07-27 04:20:52.366228105 +0900
+@@ -1,3 +1,3 @@
+ aaaa
+-日本記
++日本誘
+ bbbb
Index: test/fixtures/diffs/issue-13644-5.diff
===================================================================
--- test/fixtures/diffs/issue-13644-5.diff	(revision 0)
+++ test/fixtures/diffs/issue-13644-5.diff	(revision 0)
@@ -0,0 +1,7 @@
+--- a.txt	2013-07-27 05:52:11.415223830 +0900
++++ b.txt	2013-07-27 05:52:18.249190358 +0900
+@@ -1,3 +1,3 @@
+ aaaa
+-日本記ok
++日本誘ok
+ bbbb
Index: test/fixtures/diffs/issue-13644-3.diff
===================================================================
--- test/fixtures/diffs/issue-13644-3.diff	(revision 0)
+++ test/fixtures/diffs/issue-13644-3.diff	(revision 0)
@@ -0,0 +1,7 @@
+--- a.txt	2013-07-27 06:03:49.133257759 +0900
++++ b.txt	2013-07-27 06:03:58.791221118 +0900
+@@ -1,3 +1,3 @@
+ aaaa
+-日本記
++日本娘
+ bbbb
Index: lib/redmine/unified_diff.rb
===================================================================
--- lib/redmine/unified_diff.rb	(revision 12041)
+++ lib/redmine/unified_diff.rb	(working copy)
@@ -205,12 +205,20 @@
           end
         end
         ending = -1
-        while ending >= -(max - starting) && line_left[ending] == line_right[ending]
+        while ending >= -(max - starting) && (line_left[ending] == line_right[ending])
           ending -= 1
         end
         if (! "".respond_to?(:force_encoding)) && ending > (-1 * line_left.size)
-          while line_left[ending].ord.between?(128, 191) && ending > -1
-            ending -= 1
+          while line_left[ending].ord.between?(128, 255) && ending < -1
+            if line_left[ending].ord.between?(128, 191)
+              if line_left[ending + 1].ord.between?(128, 191)
+                ending += 1
+              else
+                break
+              end
+            else
+              ending += 1
+            end
           end
         end
         unless starting == 0 && ending == -1