diff --git a/ChangeLog b/ChangeLog
index 6e82651ee3..676453311e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Nov 19 00:41:09 2015  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* enc/euc_jp.c (mbc_case_fold): check given string is valid or not,
+	  and if invalid, return 1. [Bug #11486]
+
 Thu Nov 19 00:15:23 2015  Zachary Scott  <zzak@ruby-lang.org>
 
 	* ext/openssl/ossl_pkey.c: Merge ruby/openssl@b9ea8ef [Bug #10735]
diff --git a/enc/euc_jp.c b/enc/euc_jp.c
index 61bb8ba65a..e4d3b70167 100644
--- a/enc/euc_jp.c
+++ b/enc/euc_jp.c
@@ -381,8 +381,10 @@ mbc_case_fold(OnigCaseFoldType flag,
     OnigCodePoint code;
     int len;
 
+    len = mbc_enc_len(p, end, enc);
     code = get_lower_case(mbc_to_code(p, end, enc));
     len = code_to_mbc(code, lower, enc);
+    if (len == ONIGERR_INVALID_CODE_POINT_VALUE) len = 1;
     (*pp) += len;
     return len; /* return byte length of converted char to lower */
   }
diff --git a/test/ruby/test_m17n_comb.rb b/test/ruby/test_m17n_comb.rb
index 3a37ed4a23..83c3f8aa95 100644
--- a/test/ruby/test_m17n_comb.rb
+++ b/test/ruby/test_m17n_comb.rb
@@ -1635,4 +1635,9 @@ class TestM17NComb < Test::Unit::TestCase
     }
   end
 
+  def test_bug11486
+    bug11486 = '[Bug #11486]'
+    assert_nil ("\u3042"*19+"\r"*19+"\u3042"*20+"\r"*20).encode(Encoding::EUC_JP).gsub!(/xxx/i, ""), bug11486
+    assert_match Regexp.new("ABC\uff41".encode(Encoding::EUC_JP), Regexp::IGNORECASE), "abc\uFF21".encode(Encoding::EUC_JP), bug11486
+  end
 end
diff --git a/version.h b/version.h
index bec24f7c67..8b9a9646fa 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.2.4"
 #define RUBY_RELEASE_DATE "2015-11-19"
-#define RUBY_PATCHLEVEL 190
+#define RUBY_PATCHLEVEL 191
 
 #define RUBY_RELEASE_YEAR 2015
 #define RUBY_RELEASE_MONTH 11