Information on prepaid cards used to pay at Mos Burger restaurants and online has leaked outside the company after unauthorized access, the chain's operator Mos Food Services said. About 570,000 cards are affected.
The company said it is contacting card holders and asking them to check whether their cards show any use they do not recognize. It did not say when the unauthorized access occurred.
Prepaid cards carry stored value, so a leak of card information is a question of money as well as data: anyone able to use the details can spend the balance loaded on the card.
Mos Food Services runs the Mos Burger chain, one of Japan's largest hamburger restaurant networks. It has not said how many cards have shown unauthorized use.