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. Competition in portable gaming is heating up with the arrival of Japanese rival Sony Corp.'s latest portable offering, PlayStation Vita, which already comes with buttons that work in a similar way as the 3DS new device.

PlayStation Vita goes on sale in Japan on Dec. 17, and early next year in the U.S. and Europe.

Both 3DS and PlayStation Vita face a threat from smartphones and tablet devices that also offer on-the-go games and other entertainment. Nintendo has already slashed the price of the 3DS in a move analysts say is a sure sign it's worried about wooing consumers. Kamide says Nintendo is likely to fall short of its sales target of 16 million 3DS by the end of the fiscal year through March 2012. So far, Nintendo has sold just 4.32 million 3DS machines around the world.

Nintendo has ruled in portable gaming for years. It sold nearly 150 million DS machines since they went on sale in 2004, outpacing the Sony PlayStation Portable, whose cumulative global sales total 71 million.

But the gaming industry may be rapidly changing. "It's a bit of a hard sell," Kamide said of the 3DS attachment.