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.