There have been numerous simultaneous attempts in a short period of time to connect to the web-page that deactivated the site, according to the Kyodo news agency.
The details of and motives for the cyberattack are currently being investigated.
The incident has attracted particular attention in Japan because it happened two days before the opening of an international conference on cybersecurity, supported by the World Economic Forum in the Japanese Okinawa Prefecture.
One of the conference’s major topics will be warning against and preventing cybercrime in the run-up to and during the Olympic Games in Tokyo in 2020.
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