NTT DoCoMo Inc., Japan's biggest mobile-phone operator, and Fujitsu Ltd. said they filed a legal complaint seeking to prevent Softbank Corp. from selling a handset they claim is very similar to a model offered by DoCoMo.

The companies lodged a complaint against Softbank's 821T model made by Toshiba Corp. and known in Japan as Kantan Keitai with the Tokyo District Court on Monday, DoCoMo and Fujitsu said in a joint statement. Softbank said the two handsets are completely different.

The handset's product design closely resembles that of Fujitsu's Raku-Raku Hon model, Kiyohito Nagata, managing director of DoCoMo's product-development division, told reporters in Tokyo. "It's plainly obvious just by looking at the phones," that the position of key pad buttons and general functions are very similar, he said.

Softbank, Japan's third-largest mobile-phone operator, introduced the phone aimed at senior citizens this month to help maintain its subscriber gains lead over DoCoMo and KDDI Corp., the country's No. 2. The carriers are targeting the elderly with easy-to-use handsets as the country's aging population and the increasing ubiquity of mobile phones make the segment more important.

DoCoMo sent a warning about the design to Softbank when the Kantan Keitai was announced and then proceeded with a legal complaint because Softbank's response wasn't satisfactory, Nagata said.

"Our view on the 821T is that it's a different product from any other models aimed at senior users," Katsuhide Furuya, a spokesman for Softbank's mobile phone unit, said in a phone interview.