Two women died in Chiba and Ibaraki prefectures on Saturday as the nation experienced unseasonably hot weather, while more than 200 people were taken to hospitals with heatstroke.
Temperatures soared above 30 degrees Celsius at 315 locations nationwide, peaking at 35.6 degrees in the city of Hita, Oita Prefecture, the Meteorological Agency said.
Saturday was the hottest day on record for the month of May at more than 120 of those locations.
A 74-year-old woman died while working in a greenhouse in Chiba Prefecture and a 61-year-old woman expired while gardening in Ibaraki Prefecture, both of suspected heatstroke on Saturday morning.
The agency issued heat warnings for Miyagi and Fukushima prefectures in the northeast, Shimane and Yamaguchi prefectures in the west, and Fukuoka, Saga and Oita prefectures in the southwest.
Those taken to hospital included school students taking part in sports days or club activities outdoors.
In central Tokyo the mercury hit 31.6 degrees.
The high temperatures are set to continue Sunday, with the agency calling on people to ward off heatstroke by staying hydrated. It said the intense heat was due to a high-pressure front bringing clear skies and amplifying an already warm atmosphere.