Japanese weather officials say the unseasonably hot weather will continue in many parts of the country on Monday.
The Meteorological Agency says the temperature reached 39.5 degrees Celsius on Sunday in Saroma Town in the country's northernmost prefecture of Hokkaido, marking a national all-time high for May.
Daytime highs topped 35 degrees, around 10 degrees higher than usual, in inland areas of the Tohoku region, Tokyo and its surrounding prefectures.
Agency officials say temperatures will reach about 35 degrees in eastern and northern Japan on Monday as a warm air mass continues to dominate the regions.
The officials are urging people to use air conditioners, drink water frequently and take precautions against heat stroke.