Softbank Corp. 's shares fell as much as 5% on Wednesday afternoon, extending this week's decline, wiping out about $15 billion from the group's market value as investors worried about the group's risks in declining u.s. technology stocks.

Late last week, sources revealed that the Japanese company had made big bets on equity derivatives linked to American technology companies.

The group's share price has fallen 12 per cent since Softbank Corp. made big bets on technology-related stocks.

Sun Zhengyi, CEO of Softbank Corp. Group, said last month that he would put cash from the asset sale plan into listed stocks, but these complex transactions have caused unease among retail investors and Softbank Corp. was widely believed to be not transparent enough.

Analysts believe that Softbank Corp. bought shares while also buying call options, which allowed him to get more paper shares, which exacerbated the market rally and the subsequent sell-off.

Softbank Corp. previously declined to comment on the deals.