The Health, Labor and Welfare Ministry's website was inaccessible late Friday, and a tweet suggested the attack was apparently posted by the international group of hackers known as Anonymous.
The health ministry has since restored service and is investigating the cause.
The ministry confirmed at around 10:20 p.m. that its website had gone down in an incident similar to the overnight blackout that hit the website of the 2020 Tokyo Olympics organizing committee earlier this month. The committee said it believes the almost half-day interruption that began from the night of Nov. 4 was attributable to a cyberattack. A Twitter post hinting at a hacking attack also appeared online at that time.