The mercury skyrocketed to dangerous levels mainly in western and eastern Japan on Monday, with temperatures nearing 40 degrees Celsius in some regions.
Japan's Meteorological Agency says a high pressure system prevailed mainly over western and eastern Japan on Monday, bringing sunny weather and pushing up temperatures.
The daytime high in Shingu City, Wakayama Prefecture, western Japan, registered 39.6, while Fuchu City in Tokyo marked 39.2. Both are the highest temperatures ever recorded at this time of year since the agency began keeping track.
Dangerous heat of 38.9 degrees also hit Kuwana City, Mie Prefecture, central Japan, and Saitama City near Tokyo, had 38.6. It was 37.4 in Shizuoka City, central Japan, 36.7 in Kyoto City, 36.6 in Oita City, southwestern Japan, and 36 in central Tokyo.
As overnight temperatures are expected to be higher than 25 degrees through next morning, people are advised to keep air conditioning running at night, and to have water on hand to stay hydrated.
Scorching heat is expected again on Tuesday, with the mercury forecast to rise to 37 degrees in Saitama City and Owase City, Mie Prefecture. Likewise, a high of 36 is predicted in central Tokyo and Shizuoka City, and 35 in Kagoshima City.
As it will also remain humid, the agency and the Environment Ministry have issued heatstroke alerts for 11 prefectures from the Kanto region to Okinawa.
The prefectures under the alert are Ibaraki, Chiba, Shizuoka, Aichi, Mie, Wakayama, Tokushima, Kumamoto, Miyazaki, Kagoshima, and Okinawa except for the Miyakojima region.
Many people have suffered possible heatstroke daily and some of them have died.
People are advised to avoid going out if possible, use air conditioners appropriately, drink water and take some salt frequently.
Families are advised to pay special attention to the elderly and children who are vulnerable to heatstroke.