Coincheck posted on its website on Friday afternoon that it had suspended withdrawals of almost all cryptocurrencies.
The exchange has already reported the incident to the police and to Japan's Financial Services Agency, NHK said.
In 2014, Tokyo-based Mt. Gox, which once handled 80 percent of the world's bitcoin trades, filed for bankruptcy after losing some 850,000 bitcoins - then worth around half a billion U.S. dollars - and $28 million in cash from its bank accounts.
($1 = 108.6100 yen)
(Reporting by Makiko Yamazaki; Editing by Hugh Lawson)
