SoftBank Corp.'s stock plunged nearly 15% below the company's initial public offering price for its Tokyo Stock Exchange debut, ending Dec. 19 at ¥1,282 per share.

Shares of SoftBank Group Corp.'s Japanese mobile phone unit opened at ¥1,463 per share on Dec. 19, down from the ¥1,500 issue price. SoftBank Group raised about US$23.5 billion from the offering, making it Japan's biggest IPO and just below Chinese e-commerce giant Alibaba Group Holding Ltd.'s record US$25 billion listing on Wall Street in 2014, Reuters reported.

"It was beyond our expectations that the shares would fall that much," a senior executive at one of SoftBank Corp.'s domestic lead underwriters reportedly said.

As of Dec. 18, US$1 was equivalent to ¥112.48.