SoftBank Corp. said Friday it will merge its four domestic telecommunications subsidiaries next April to improve business efficiency and competitiveness.

The telecoms and Internet giant will integrate its operations under the surviving SoftBank Mobile Corp. starting April 1. The SoftBank board approved the structural reform earlier in the day.

The three other firms involved in the merger are SoftBank BB Corp., which provides broadband services under the "Yahoo BB" brand, fixed-line operator SoftBank Telecom Corp. and Ymobile Corp.