Weather officials say the heaviest rainfall in half a century has hit Tsushima City, Nagasaki Prefecture, as severe tropical storm Tapah moves through the region.
Hourly rainfall of 80 millimeters was recorded on Sunday evening in the city, which is an island on the Sea of Japan.
City officials say they've received reports of flooding from residents, but they are unable to grasp the situation in detail.
The officials are not instructing residents to evacuate because it's dangerous to go outside at night in heavy rain and strong winds.
They are using the disaster radio system to urge people to stay indoors, move to upper floors and to keep away from mountain slopes to protect themselves from mudslides.