Softbank Corp., the nation's third-biggest mobile carrier, said Thursday that it sank into a loss in the January-March quarter due to one-time costs, including the launch of a superfast Internet service.

Softbank reported a net loss of ¥15.01 billion for the fiscal fourth quarter, a reversal from the ¥15.4 billion it earned the same period the previous year.

Quarterly sales dipped 3.7 percent to ¥690.8 billion as the recession cut into handset sales.

Softbank has been more successful than its rivals, NTT DoCoMo and KDDI Corp., in wooing people to its cell phone services for 23 straight months through March with discounts and by being the exclusive local carrier for eye-catching handsets like Apple Inc.'s iPhone.

But it has yet to translate its aggressive marketing campaigns, including TV ads featuring a popular talking white dog, into solid profit gains.

Softbank, now with about 20.6 million subscribers, controls about 19.2 percent of the market, up 1.1 percentage points from the previous year. But average sales per user fell for voice calls and rose for data transmission.

Losses on investments caused by the market downturn dragged down earnings, said Softbank, which bought British cellular giant Vodafone Group PLC's struggling Japanese operations in 2006.

A major one-time loss related to payments for bonds for its mobile unit as well as a writeoff for its fiber-optic Internet services, also hurt results.

One business area that performed better than last year was its Internet-related "cultural" businesses, including advertising and Internet shopping.