Several thousand people have been evacuated from their homes due to the flooding exacerbated by Typhoon Nanmadol. Rescue operations are continuing and the death toll may rise.
Thousands of Japanese rescuers searched on Friday for victims of freak rains that have killed at least 11 people and left hundreds cut off from the outside world by landslides.
Torrential rain hit southwest Japan on Wednesday and was still moving north on Friday.
According to the latest figure, more than 140,000 people were under orders to evacuate their homes while authorities issued new warnings of landslides across the island of Kyushu.
Parts of Fukuoka prefecture were hit by 600.5 mm (24 inches) of rain in the 48 hours to 10:40 am on Friday (0140 GMT), more than 1.5 times the usual rain for the month, the meteorological agency said.
Eleven people had been killed while 14 were unaccounted for, public broadcaster NHK said.
"It came in from the back," one man told NHK as he gazed at his house, shattered by a surge of mud and wood. The whole area has been buried. There aren't any words for this," the man said in a quivering voice.
About 12,300 soldiers, policemen and firemen clambered across expanses of debris of splintered wood and mud hoping to reach about 500 people cut off by landslides, NHK reported.
Large boulders and uprooted trees blocked streets as residents with heavy bags picked their way carefully through their neighbourhoods.
"At first, it wasn't raining that much," said Sumie Umeyo, a resident of Asakura city.
"But they spoke of record-breaking rain and it started raining heavily, then they began closing the roads. We looked outside and the roads were like rivers," Umeyo said.
Fukuoka and neighbouring Oita, the hardest hit areas, are both largely rural but rivers were also rising in the city of Kitakyushu, which has a population of some 950,000 and issued evacuation orders for several districts.
The rain was caused by a low pressure area over the Pacific that fed warm, moist air into Japan's seasonal rainy front.
The rains came after after Typhoon Nanmadol slammed Japan on Tuesday which dumped rain that has wrecked homes, roads and rice terraces.
Amid search and rescue efforts, Japan's royal family decided to postpone Saturday's formal announcement of Princess Mako's engagement with her college classmate out of consideration for the suffering of people in the affected areas, Japanese media reported.