Severe tropical storm Krosa is moving northwest over waters south of Japan. It could make landfall somewhere in the country by Thursday.
The Meteorological Agency warns that western and eastern Japan could see heavy rains even before the storm nears land.
The storm is expected to further strengthen as it comes close to western Japan on Wednesday and Thursday.
Strong winds are forecast along the Pacific coast of western and eastern Japan. Winds of up to 108 kilometers per hour are expected in southern Kyushu.
There are already high waves across wide areas. Waves up to 10 meters are forecast for Shikoku through Wednesday.
Over the 24-hour period to Wednesday morning, 300 millimeters of rain are forecast for the Tokai region, 200 millimeters for Kansai and 100 to 150 millimeters for Kyushu and Shikoku.
Over the 24 hours to Thursday morning, 400 to 600 millimeters of rain are expected in Shikoku, 300 to 500 millimeters in Kyushu and Kansai, and 300 to 400 millimeters in Tokai.
Weather officials say some areas could see total rainfall of more than 1,000 millimeters.
They are warning of stormy conditions during the traditional Bon summer holidays, when many people head for their hometowns and other destinations.
Caution is also advised for strong winds, high waves and storm surges. The agency is also warning there is a risk of landslides or other disasters.