The number of Self-Defense Forces members to be deployed in Kumamoto Prefecture and other areas hit by the downpour will be doubled to 20,000.
Rescue workers have been mobilized to search for those missing and help people evacuate. At least 12 people remain unaccounted for since the rains first pounded the prefecture over the weekend, causing flooding and mudslides.
About 1.38 million people in the region and nearby provinces have been advised to seek refuge.
More rain is expected through Wednesday and onwards, across wide areas from Southwestern to Northeastern Japan.
In the city of Anshun in Southwest China’s Guizhou province, a bus carrying students to an exam plunged into a reservoir on Tuesday after making a sudden swerve across five other lanes of traffic, leaving 21 dead and 16 injured.
Days of torrential rains have triggered flood warnings across large parts of the country and disrupted the first day of national college exams.