Tokyo fire department officials say 53 people in the capital had been taken to hospital for suspected heatstroke by 3 p.m. on Wednesday.
The officials say a man in his 80s in Kokubunji City was found collapsed in his home and rushed to hospital in critical condition.
Of the other 52 people, two in their 50s and 70s have shown serious symptoms, 25 in their 20s to 90s moderate symptoms, and the remaining 25 in their 20s to 90s mild symptoms.
Weather forecasters say the scorching heatwave will ease slightly on Thursday as the high in the capital is expected to be 31 degrees Celsius — nearly 3 degrees lower than Wednesday.
Officials are calling on residents to take liquids before they feel thirsty, stay indoors and use air conditioning and electric fans accordingly.