Fast Retailing Co. Chief Executive Officer Tadashi Yanai is leaving SoftBank Group Corp.'s board after more than 18 years as one of the few directors with the heft to challenge Masayoshi Son.

Yanai, Japan's richest man, will step down on Tuesday, according to a SoftBank statement Friday. He is leaving the post to focus on running his own business, SoftBank spokeswoman Hiroe Kotera said. The founder of fashion chain Uniqlo has served as a SoftBank board member since June 2001.

With this departure, SoftBank investors lose one of the few board members capable of standing up to Son. The 70-year-old has been reported as a rare voice of dissent when it came to Son's ambitious and risky acquisitions. The two men, whose respective companies both went public in the same month in 1994, have often engaged in jocular sparring at SoftBank's annual shareholder meetings.