Intense heat of 35 degrees Celsius or higher was recorded across much of Japan on Friday, with the mercury approaching 40 degrees in a central city.
The Japan Meteorological Agency says a high-pressure system over the Pacific covered wide areas of the country's east and west, raising temperatures.
At a measuring spot in Matsusaka City in the central prefecture of Mie, the daytime high hit 39.7 degrees Celsius. That is the highest temperature observed there since the agency began taking records in 1979.
Obama City in Fukui Prefecture on the Sea of Japan coast had a high of 38.9 degrees, and Kuroshio Town in the western prefecture of Kochi 38.0 degrees.
Scorching heat of 35 degrees or higher was recorded at more than 160 observation points in wide areas from northeastern Japan to the southern prefecture of Okinawa.
In Tokyo, a total of 55 people between the ages of 23 and 90 had been taken to hospitals for possible heatstroke by 3 p.m., according to the Tokyo Fire Department.
Weather officials forecast dangerously hot conditions again on Saturday. They expect highs of 38 degrees in the central city of Shizuoka, and 35 degrees or more in many cities including Kofu and Miyazaki.
The officials say temperatures may rise even further on Sunday, mainly in the Kanto and Tokai regions.
People are advised to avoid exercising outdoors and non-essential outings, and to use air conditioners properly.