The mercury topped 40 degrees Celsius in Shizuoka Prefecture, central Japan, on Monday morning.
Japanese weather officials say a high pressure system is bringing sunny weather to western and eastern Japan, driving up temperatures from the morning.
The temperature in Shizuoka’s Hamamatsu City was 40.2 degrees shortly after 11:40 a.m.
By 11:30 a.m., the mercury had risen to 38.7 degrees in Sano City, Tochigi Prefecture; 38.3 in Tokyo’s Hachioji City; and 38.2 in Kiryu City, Gunma Prefecture.
Daytime highs are expected to be 41 degrees in Hamamatsu and 40 in Saitama Prefecture’s Kumagaya City, Yamanashi Prefecture’s Kofu City and Gunma’s Maebashi City.
Heatstroke alerts have been issued for 38 out of 47 prefectures.
People are advised to take thorough protective measures. They include avoiding non-essential outings, not hesitating to use air conditioning and frequently taking fluids and salt.
Close attention should be paid to elderly people and infants because they are at higher risk of developing heatstroke.