At least 131 people died of heatstroke in Tokyo this month.
Majority of the fatalities occurred indoors. Over 80 percent of the victims were aged at least 70 years or above.
People wearing masks to protect themselves from coronavirus also contributed to the increasing rate of heatstroke cases.
Officials said elderly were particularly susceptible to heat-related illnesses, with those living alone at particular risk.
Public health authorities are advising people to use air conditioning around the clock, and to drink water frequently to help prevent heatstroke.
Officials asked Tokyoites to take warnings seriously as more people died of heatstroke than coronavirus recently.
Over 14 million people live in Tokyo.