diff --git a/autopep8.py b/autopep8.py
index feb46634..9efd3b41 100755
--- a/autopep8.py
+++ b/autopep8.py
@@ -1295,8 +1295,14 @@ def fix_w503(self, result):
                 before_line[:comment_index], one_string_token,
                 before_line[comment_index + 1:])
         else:
-            self.source[fix_target_line] = '{} {}{}'.format(
-                before_line[:bl], one_string_token, before_line[bl:])
+            if before_line[:bl].endswith("#"):
+                # special case
+                # see: https://github.com/hhatto/autopep8/issues/503
+                self.source[fix_target_line] = '{}{} {}'.format(
+                    before_line[:bl-2], one_string_token, before_line[bl-2:])
+            else:
+                self.source[fix_target_line] = '{} {}{}'.format(
+                    before_line[:bl], one_string_token, before_line[bl:])
 
     def fix_w504(self, result):
         (line_index, _, target) = get_index_offset_contents(result,
diff --git a/test/test_autopep8.py b/test/test_autopep8.py
index 5f6e0213..8cb9b605 100755
--- a/test/test_autopep8.py
+++ b/test/test_autopep8.py
@@ -4452,6 +4452,22 @@ def test_w503_with_comment_double(self):
     22222222 and  # C2
     333333333333  # C3
 )
+"""
+        with autopep8_context(line, options=['--select=W503']) as result:
+            self.assertEqual(fixed, result)
+
+    def test_w503_with_comment_with_only_comment_block_charactor(self):
+        line = """\
+if (True  #
+    and True
+    and True):
+    print(1)
+"""
+        fixed = """\
+if (True and  #
+    True and
+    True):
+    print(1)
 """
         with autopep8_context(line, options=['--select=W503']) as result:
             self.assertEqual(fixed, result)