Yahoo Japan Corp. has agreed to turn big online fashion retailer Zozo Inc. into a subsidiary for as much as ¥400 billion, which is expected to help it compete more effectively with larger rivals, such as Amazon Inc. and Rakuten Inc.

The deal is also likely to bolster the e-commerce prospects of Yahoo Japan, whose biggest shareholder is SoftBank Corp., the domestic telecom business of Masayoshi Son's SoftBank Group Corp. empire.

The company offered ¥2,620 per share, a 21 percent premium on Zozo's closing price, sending its shares soaring Thursday.