Softbank Corp. said Thursday it has agreed to buy Japan Telecom Co. from U.S. investment fund Ripplewood Holdings LLC for about 340 billion yen, including the assumption of debts.

The announcement came only six months after Ripplewood acquired the company from Vodafone Group PLC of Britain for some 260 billion yen.

With the acquisition of Japan's third-largest fixed-line operator, Softbank will have nearly 900 billion yen in annual revenues and move a step closer to becoming a serious challenger to communications industry behemoth NTT Corp.