The number of confirmed deaths after recent severe storms across Japan rose above 200 on Thursday, local media reported.
Dozens also remain missing, the reports said.
The country's west was lashed by heavy storms last week and during much of the weekend, bringing flooding and landslides which led to Japan's worst natural disaster since the 2011 tsunami and earthquake that prompted the Fukushima nuclear accident.
More than 200,000 households had no water a week after torrential rains caused floods and set off landslides across western Japan, bringing death and destruction to decades-old communities built on mountain slopes and flood plains.
With daily temperatures above 30 Celsius (86 Fahrenheit) and high humidity, life in school gymnasiums and other evacuation centers, where families spread out on mats on the floors, began to take a toll.
Television footage showed one elderly woman trying to sleep by kneeling with her upper body on the seat of a folding chair, arms over her eyes to keep out the light.
With few portable fans in the evacuation centers, many survivors tried to cool themselves with paper fans.
The limited water supply meant that people are not getting enough fluids and in danger of suffering from heatstroke, authorities said. People are also reluctant to use what water they do have to wash their hands, raising fears of epidemics.
"Without water, we can't really clean anything up. We can't wash anything," one man told NHK television.
The government has sent water trucks to the disaster area, but supplies remain limited.
More than 70,000 military, police and firefighters toiled through the debris in a grim search for the missing.
Some teams shoveled dirt into sacks and piled the bags into trucks. Others used diggers and chainsaws to work through landslides and splintered buildings.
Many areas were buried deep in mud that smelled like sewage and had hardened in the heat, making the search more difficult.
Disasters set off by torrential rains have become more frequent in Japan, perhaps due to global warming, experts say. Dozens of people died after similar rains caused flooding around the same time last year.
"It's an undeniable fact that this sort of disaster due to torrential, unprecedented rain is becoming more frequent in recent years," Chief Cabinet Secretary Yoshihide Suga said at a news conference in Tokyo.
"Preserving the lives and peaceful existence of our citizens is the government's biggest duty. We recognize that there's a need to look into steps we can take to reduce the damage from disasters like this even a little bit," he added.