The Coincheck exchange said on its website Friday that it had halted sales and withdrawals of the currency, which is called NEM. It later added that it had restricted dealings in most other cryptocurrencies too.
At a Friday night news conference, Coincheck President Koichiro Wada bowed and apologized. He said the company may seek financial assistance, according to Kyodo News service. Japanese TV footage showed a small group of customers standing outside the company's Tokyo head office Friday night.
Coincheck, which calls itself the leading Bitcoin and cryptocurrency exchange in Asia, said it detected the unauthorized access to its system about 3 a.m. Friday.
The reported loss tops the 48 billion yen that Mt. Gox, a Japan-based Bitcoin exchange, lost in 2014.
--The Associated Press