Scorching temperatures were recorded nationwide Tuesday, with readings topping 35 C measured across the country.
The Japan Meteorological Agency called for vigilance against heatstroke, and advised people to refrain from going outdoors.
According to the agency, the highest temperatures recorded by 3 p.m. included 39.5 C in Koshigaya, Saitama Prefecture, and 38.8 C in Tajimi, Gifu Prefecture and Ome City, Tokyo.
Inland areas of the Kanto and Tokai regions were especially affected.
The JMA and the Environment Ministry issued a heatstroke alert — which is announced on days when there is a significant risk of developing heatstroke — for Tokyo and 36 prefectures in and west of the Kanto region, the largest number this year.