SoftBank’s stock plummeted by almost 19%, resulting in a $4.6 billion decrease in the wealth of its founder, Masayoshi Son.

On Monday, SoftBank Group’s shares plummeted nearly 19%, significantly reducing the wealth of its founder, Masayoshi Son, by billions of dollars. This drop occurred amid a global market downturn, following the Bank of Japan’s decision to increase its benchmark interest rate, which triggered a decline in Japanese stocks starting last Thursday.

The Nikkei 225 index experienced a severe 12.4% drop, marking its worst performance since the 1987 “Black Monday” crash. As a result, Son’s net worth decreased by $4.6 billion in a single day, according to Forbes’ real-time billionaires list.

Despite a strong performance earlier this year, where SoftBank’s stock reached a record high due to the recovery of its Vision Fund and a significant rise in the share price of Arm, a British chip designer largely owned by SoftBank, the recent downturn has left the company’s shares only 1.7% higher for the year. CNBC’s calculations indicate that SoftBank’s market value has decreased by approximately $28.3 billion since the close of trading last Wednesday.

Investors are now looking forward to SoftBank’s fiscal first-quarter earnings report, set to be released on Wednesday, hoping for positive signs of continued recovery in the Vision Fund. Masayoshi Son, who had been keeping a low profile, reemerged in June to share his ambitious vision for the future of artificial intelligence, predicting it will become 10,000 times more intelligent than humans.