Daytime temperatures are expected to soar mainly in eastern and western Japan, with the mercury likely rising to dangerous levels in Kinki and some other regions.
Authorities are urging people to take thorough measures to protect against heatstroke.
The Japan Meteorological Agency says a high-pressure system will bring clear skies to much of Japan, from the Tohoku region to down to Okinawa Prefecture and the Amami region in Kagoshima Prefecture.
Temperatures are likely to rise across the country. The daytime high is forecast at 39 degrees Celsius in Kyoto City.
The mercury is set to hit 38 degrees in the cities of Nagoya, Osaka as well as Kagawa Prefecture’s Takamatsu City and Oita Prefecture’s Hita City.
Nagano Prefecture’s Kofu City and Okayama City are likely to reach 37 degrees.
Fukushima Prefecture’s Aizuwakamatsu City as well as Fukui and Fukuoka cities are expected to see a daytime high of 36 degrees.
Heatstroke alerts have been issued for 34 out of 47 prefectures, from the Kanto-Koshin region to Okinawa Prefecture.
People are advised to avoid non-essential outings, use air conditioners without hesitation and consume adequate amounts of fluids and salt on a regular basis.
Special attention should be paid to elderly people and infants because they are at higher risk of developing heatstroke.