Softbank Corp. posted a record high operating profit of 271 billion yen in its business year to last March, up from 62.3 billion yen the year before thanks to its purchase of British cell phone giant Vodafone Group PLC's struggling Japan unit in April 2006.

Annual group sales more than doubled to 2.54 trillion yen. Net profit fell 49.9 percent to 28.8 billion yen due to higher taxes and smaller extraordinary incomes.

"Purchasing Vodafone was a success," Softbank CEO Masayoshi Son told a news conference.

Softbank's mobile phone unit, Softbank Mobile Corp., has become the core earnings unit for the company, with its operating profit accounting for 57 percent of the group's operating profit in the reporting year.

The mobile phone unit's operating profit was 155.7 billion yen.

"The mobile phone business is performing well," Son said. "Since the number portability system was introduced last October, we continued to increase net subscribers."

Under the number portability system, people can keep the same phone number when they change service providers.

Son said his company had feared that their customers might switch to rivals NTT DoCoMo Inc. or KDDI Corp., but Softbank Mobile had more new subscriptions than cancellations since number portability was introduced. The company added a net 160,000 subscribers last month, he said.