Japanese media said on Friday that a cryptocurrency exchange has lost 58 billion yen (530 million US dollars) because of hacking.
The Coincheck exchange said on its website that it had halted sales and withdrawals of a cryptocurrency called NEM.
It later added that it had restricted dealings in most other cryptocurrencies too.
Coincheck President Koichiro Wada apologised at a news conference and said the company may seek financial assistance.
The Kyodo News service said the Coincheck exchange detected the unauthorised access to its system early on Friday.
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The reported loss tops the 48 billion yen (441.6 million US dollars) that the Japan-based Mt. Gox Bitcoin exchange lost in 2014.
