diff --git a/mrbgems/mruby-socket/mrblib/socket.rb b/mrbgems/mruby-socket/mrblib/socket.rb
index 70785363a9622d7fd539d236b9bbd592721fb809..7f0dc7982ff4d4fc7369c1b10c7e87b74b83c57e 100644
--- a/mrbgems/mruby-socket/mrblib/socket.rb
+++ b/mrbgems/mruby-socket/mrblib/socket.rb
@@ -178,7 +178,7 @@ class BasicSocket < IO
 
   def initialize(*args)
     super(*args)
-    self.is_socket = true
+    self._is_socket = true
     @do_not_reverse_lookup = @@do_not_reverse_lookup
   end
 
diff --git a/mrbgems/mruby-socket/src/socket.c b/mrbgems/mruby-socket/src/socket.c
index 937c33011f97cf2375ae2267d0390550b6e1e0bb..a4e1d483a2ff72a03f5b89ba22b1764151a4c843 100644
--- a/mrbgems/mruby-socket/src/socket.c
+++ b/mrbgems/mruby-socket/src/socket.c
@@ -856,7 +856,7 @@ mrb_mruby_socket_gem_init(mrb_state* mrb)
   // #sendmsg_nonblock
   mrb_define_method(mrb, bsock, "setsockopt", mrb_basicsocket_setsockopt, MRB_ARGS_REQ(1)|MRB_ARGS_OPT(2));
   mrb_define_method(mrb, bsock, "shutdown", mrb_basicsocket_shutdown, MRB_ARGS_OPT(1));
-  mrb_define_method(mrb, bsock, "is_socket=", mrb_basicsocket_set_is_socket, MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, bsock, "_is_socket=", mrb_basicsocket_set_is_socket, MRB_ARGS_REQ(1));
 
   ipsock = mrb_define_class(mrb, "IPSocket", bsock);
   mrb_define_class_method(mrb, ipsock, "ntop", mrb_ipsocket_ntop, MRB_ARGS_REQ(1));