SoftBank shares were trading at 5,122 yen at 10:39 a.m. (0139 GMT), down by half from March highs and at the lowest level in 15 months.
"What you've seen is almost a complete reversal in sentiment," said analyst Kirk Boodry at Redex Research.
"All of the underlying value is being squeezed."
Shares in Chinese e-commerce firm Alibaba, the group's most valuable investment, fell 8% in New York on Friday. That came after ride-hailer Didi, one of the Vision Fund's top investments, said it would delist in the U.S. following pressure from Chinese regulators.
CEO Masayoshi Son lamented the group's conglomerate discount at earnings last month, pointing to upside including the planned sale of chip designer Arm to Nvidia.
However the U.S. Federal Trade Commission said on Thursday it will sue to block the deal, in the latest case of SoftBank struggling to get deals through regulatory scrutiny.
The slide comes even as SoftBank buys back its shares in a 1 trillion yen ($8.85 billion) programme launched last month.
The group has signalled the pace of repurchases could be slower than during a previous record 2.5 trillion buyback.
($1 = 113.0000 yen)
(Reporting by Sam Nussey; Editing by Stephen Coates)