Nintendo Co. shares closed at 16,970 yen ($212.44), down more than 5 percent. The benchmark Nikkei 225 index ended flat.
Shuji Hosoi, analyst at Daiwa Securities Co., said it was unclear how successfully the machine would compete against smartphones and tablet PCs, when device-based gaming was already having to vie against social networks.
It is hard to see how it was different enough to woo users of smartphones and tablet PCs back to gaming, he said.
"People are puzzled whether this will really sell."
"But it would be extremely difficult because the competition is so intense," he said, referring to products such as the iPad from Apple Inc. and other rivals. "People have already changed."
The Japanese gaming giant behind Pokemon and Super Mario games said the Wii U will broadcast high-definition video and feature a touchscreen controller that can detect motion. Its price was not disclosed.
Nintendo said the Wii U will be released between April and December next year and will be compatible with older Wii games and controllers.