Masayoshi Son, chairman and chief executive officer of SoftBank Group Corp., speaks during a news conference in Tokyo, Japan, on Thursday, Oct. 4, 2018. Japan's??SoftBank??and??Toyota Motor Corp.??are teaming up on ride-hailing and self-driving cars as they accelerate their push into a market dominated by U.S. technology and car companies. Photographer: Kiyoshi Ota/Bloomberg via Getty Images

Softbank’s blockbuster IPO has flopped in its market debut.Shares in the Japanese company’s mobile telecommunications unit plunged as much as 10% in Tokyo on Wednesday, their first day of trading.

The business is one of Japan’s biggest wireless carriers and has provided the foundation for SoftBank CEO Masa Son’s vast tech empire. By listing a big chunk of it, SoftBank Group (SFTBY) has raised about $23.5 billion.

That makes it Japan’s biggest ever stock float — and the world’s largest since Alibaba’s (BABA) $25 billion listing in New York in 2014.

Strong demand from investors for a piece of the mobile business had prompted Softbank to increase the number of shares it was selling.But it went public at a tough time for stocks: Japan’s benchmark Nikkei index has fallen more than 6% since the start of December and is down about 13% from its recent high in early October.

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