Softbank's blockbuster IPO has flopped in its market debut.

Shares in the Japanese company's mobile telecommunications unit plunged 15% 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.

SoftBank's mobile business has also faced some negative headlines this month. It suffered an embarrassing service outage on December 6 and said last week it's reviewing whether to remove equipment made by Chinese manufacturer Huawei from its network, a potentially costly move.

Japanese mom-and-pop investors, who bought up most of the shares in the IPO, were dumping the stock on the first day partly over fears of a damaging price war, according to analysts.

Japan's biggest wireless carrier, NTT Docomo (DCMYY), will slash the prices of its cellphone plans by as much as 40% next year after pressure from the Japanese government. Softbank is expected to follow suit.

Travis Lundy, an analyst at investment research platform Smartkarma, said the main reason for investors to buy the stock is because it pays a 5% dividend, which is large by Japanese standards. If a price war hits earnings, that dividend could come under threat.

Listing the mobile business is a key part of Son's efforts to reposition SoftBank as a global tech investor. The move splits the company in two, allowing investors to choose between its mobile unit and its tech investment business, which includes a big stake in Alibaba.

Son has become a major player in the global tech industry after launching the almost $100 billion Vision Fund last year. (CNN)

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