Photo/IllutrationSoftBank Group Corp. Chief Executive Officer Masayoshi Son, left, speaks during a SoftBank World presentation at a hotel in Tokyo on July 20, 2017. (AP file photo)

SoftBank Group Corp.'s Japanese mobile subsidiary began trading on the Tokyo Stock Exchange on Wednesday in one of the world's biggest share offerings.

Five executives each rang a bell with a wooden hammer to celebrate the IPO that seeks to raise more than 2 trillion yen ($18 billion). They didn't speak at the ceremony but are expected to announce the first-day results at a news conference after the market closes.

SoftBank shares fetched an initial price of 1,463 yen ($13.03) and by midmorning, slid further to 1,392 yen, down 7 percent from the IPO price of 1,500 yen announced earlier this month.

The listing came just weeks after massive service outage attributed to a software problem, affecting a large number of customers. They were unable to text messages or make payments during the outage.

SoftBank is listing 1.6 billion shares, or about one-third held by its parent company.

The IPO compares with some of the world's biggest. China's Alibaba Group raised about $20 billion when it went public in 2014, and Facebook raised $16 billion in 2012.

SoftBank Group's chief, Masayoshi Son, has also drawn investor concerns for his relations with Saudi Arabian Crown Prince Mohammed bin Salman after the killing of Saudi journalist Jamal Khashoggi.

About half of SoftBank Group's $100 billion Vision Fund investment money comes from the kingdom. The fund has been investing in solar projects and artificial intelligence.

The parent SoftBank wants to add cash for its investments not linked to Saudi Arabia.

It has invested in a range of companies globally, such as U.S. wireless company Sprint, British IoT company ARM and Chinese e-commerce Alibaba.

SoftBank, which created the Pepper companion robot, was founded in 1986, as a software, broadband and fixed-line telecommunications company. It was the first mobile carrier to offer the Apple iPhone in Japan.