diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 560acde62..eeb3c7f86 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -139,8 +139,7 @@ io_modestr_to_flags(mrb_state *mrb, const char *mode)
       flags = O_WRONLY | O_CREAT | O_APPEND;
       break;
     default:
-      mrb_raisef(mrb, E_ARGUMENT_ERROR, "illegal access mode %s", mode);
-      flags = 0; /* not reached */
+      goto modeerr;
   }
 
   while (*m) {
@@ -150,17 +149,25 @@ io_modestr_to_flags(mrb_state *mrb, const char *mode)
         flags |= O_BINARY;
 #endif
         break;
+      case 'x':
+        if (mode[0] != 'w') goto modeerr;
+        flags |= O_EXCL;
+        break;
       case '+':
         flags = (flags & ~OPEN_ACCESS_MODE_FLAGS) | O_RDWR;
         break;
       case ':':
         /* XXX: PASSTHROUGH*/
       default:
-        mrb_raisef(mrb, E_ARGUMENT_ERROR, "illegal access mode %s", mode);
+        goto modeerr;
     }
   }
 
   return flags;
+
+ modeerr:
+  mrb_raisef(mrb, E_ARGUMENT_ERROR, "illegal access mode %s", mode);
+  return 0; /* not reached */
 }
 
 static int
diff --git a/mrbgems/mruby-io/test/file.rb b/mrbgems/mruby-io/test/file.rb
index 419b40e23..5746986ad 100644
--- a/mrbgems/mruby-io/test/file.rb
+++ b/mrbgems/mruby-io/test/file.rb
@@ -260,4 +260,30 @@ assert('File.chmod') do
   end
 end
 
+assert('File.open with "x" mode') do
+  File.unlink $mrbtest_io_wfname rescue nil
+  assert_nothing_raised do
+    File.open($mrbtest_io_wfname, "wx") {}
+  end
+  assert_raise(RuntimeError) do
+    File.open($mrbtest_io_wfname, "wx") {}
+  end
+
+  File.unlink $mrbtest_io_wfname rescue nil
+  assert_nothing_raised do
+    File.open($mrbtest_io_wfname, "w+x") {}
+  end
+  assert_raise(RuntimeError) do
+    File.open($mrbtest_io_wfname, "w+x") {}
+  end
+
+  assert_raise(ArgumentError) do
+    File.open($mrbtest_io_wfname, "rx") {}
+  end
+
+  assert_raise(ArgumentError) do
+    File.open($mrbtest_io_wfname, "ax") {}
+  end
+end
+
 MRubyIOTestUtil.io_test_cleanup