SoftBank Corp shares have tumbled more than 14 per cent on debut, as investor appetite for Japan's biggest ever IPO was hurt by a recent service outage at the telecoms operator and worries over its exposure to Chinese telecoms gear maker Huawei.

The poor start on Wednesday for the unit of investment giant SoftBank Group Corp meant that for Japan's mum and dad investors concerns about the company and the nation's telecoms market trumped the appeal of the group's charismatic founder Masayoshi Son.

Such a debut is also uncommon in the Japanese IPO market. Of 82 IPOs so far this year, SoftBank Corp's $US23.5 billion float was only the seventh to open below the offering price. Among recent major IPOs, Japan Display was the only one to flop, suffering a fall in its 2014 debut.

"There was a disruption in its network early this month as well as Huawei's issues. There hasn't been good news involving SoftBank recently," said Tetsuro Ii, chief executive officer at Commons Asset Management.

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The IPO was just shy of the world record $US25bn 2014 listing of Alibaba

Shares of SoftBank Corp closed at 1282 yen, or 14.5 per cent lower than its IPO price of 1500 yen. They opened at 1463 yen.

SoftBank Corp shares were the most heavily traded on the Tokyo Stock Exchange's first section.

SoftBank Group lost 0.9 per cent and the broader Tokyo market eased 0.4 per cent.

The IPO was just shy of the world record $US25 billion 2014 listing of Chinese e-commerce giant Alibaba Group Holding, a SoftBank Group portfolio company.

SoftBank Group raised 2.65 trillion yen ($US23.5 billion) in the IPO. It will retain about 63 per cent of the newly listed unit should a green shoe option be exercised in full. The IPO is a milestone in the conglomerate's transformation into primarily a global tech investor.

During the IPO period, Japan's third-largest mobile phone network provider by subscriber numbers suffered a rare nationwide service outage, which it said would not affect earnings or dividends.

Adding to investor worries, SoftBank Corp's relationship with Huawei Technologies Co Ltd came under scrutiny as governments around the world moved to shut out the Chinese firm amid worries its gear could facilitate Chinese spying.

SoftBank Corp plans to replace Huawei-provided 4G network equipment with other suppliers' hardware, two sources said, in a process likely to be time-consuming and expensive.