Intense heat is expected in Japan from the western region to the eastern region on Sunday.
Some areas in Kyushu are expected to see the maximum temperatures near 40 degrees Celsius.
People are being advised to avoid exercising or going outside as much as possible.
The Meteorological Agency says that the mercury rose to 39.8 degrees in Shimanto City in Kochi Prefecture on Saturday.
It says that temperatures higher than 35 degrees were recorded at 292 observation points, the largest number this summer.
Weather officials say that a high pressure system over eastern and western Japan is likely to drive up temperatures on Sunday.
Daytime highs are forecast to reach 39 in Kurume City in Fukuoka Prefecture, 38 in Kumamoto City and Saga City.
And the mercury is expected to rise to 37 in Osaka City, Nagoya City, Takamatsu City and Kofu City in Yamanashi Prefecture while 36 degrees are forecast in Okayama City and Maebashi City in Gunma Prefecture.
Daytime highs are forecast to reach 35 in central Tokyo and Yamagata City.
The meteorological agency and environment ministry have issued heatstroke alerts for 37 of Japan’s 47 prefectures.
They are Fukushima, Ibaraki, Tokyo, Kanagawa, Nagano, Yamanashi, Shizuoka, Aichi, Gifu, Mie, Niigata, Toyama, Ishikawa, Fukui, Shiga, Kyoto, Osaka, Hyogo, Nara, Wakayama, Okayama, Hiroshima, Shimane, Tottori, Tokushima, Kagawa, Ehime, Kochi, Yamaguchi, Fukuoka, Oita, Nagasaki, Saga, Kumamoto, Miyazaki, Kagoshima and Okinawa, especially on the main island of Okinawa.
In addition to refraining from exercising and going outside unless necessary, people are advised to use air conditioning indoors and to regularly consume adequate amounts of fluids and salt.
As it is summer holiday season, during outdoor leisure activities, it is advisable to carry ice in water bottles or ice packs.
People are asked to keep an eye on infants and the elderly, who are at higher risk of developing heatstroke.
Weather officials say atmospheric conditions are unstable in the Kinki and Chugoku regions with warm and damp air flowing in, and could cause thunder storms in some areas.
They say torrential rain with lightning may fall in some local areas from Tohoku to western Japan in the afternoon.
The meteorological agency is advising people to remain alert for sudden, heavy rain, lightning strikes and gusts including tornadoes.