YURI KAGEYAMATOKYO (AP) â¿¿ Nintendo's add-on button device for its 3DS video game machine is facing criticism as a possible design flaw and a detraction for the already struggling portable. Some critics say Nintendo Co. is embarrassed about the 1,500 yen ($20) "slide pad," noting President Satoru Iwata didn't mention a word about the gadget at his presentation earlier this week. The device, which goes on sale in Japan on Dec. 10, works as another controller. It's handy to have for more complex games from outside software makers like the "Monster Hunter" series from Capcom Co., set to go on sale in Japan on the same day. Five other games, including "Metal Gear Solid Snake Eater 3D" from Konami Corp., also can use the extra device. More games may be added to the list. Nintendo, the Japanese manufacturer of the Wii home console and Super Mario and Pokemon games, says the peripheral part isn't necessary, just a matter of preference. Adding it helps deliver a gaming experience more similar to a home console in remote-controlling. Hiroshi Kamide, analyst at JPMorgan Securities Japan Co., believes Nintendo is displaying a surprising lack of confidence. "It's a bit of a shame. Only six months after releasing a new platform they're having to come out with an attachment to make it more attractive for some people," he said. Game fans were disappointed. The part, although not massive, makes 3DS bulky and harder to carry around in your pocket, important for a portable. No wonder the device was quietly introduced on the company website this week. Iwata said nothing about the device during his presentation, although he delivered praise about the upcoming "Monster Hunter" and other games to a packed crowd at a Tokyo event. The device has also been witnessed, but not displayed prominently, at the Capcom booth at the Tokyo Game Show exhibition, which previewed to media Thursday and opens to the public this weekend.