A total of 12,666 heatstroke patients were taken to hospital around Japan in the week through Sunday, when temperatures reached 39 degrees Celsius in many places in the country.
Data from the Fire and Disaster Management Agency showed that the number went up more than 3,500 from the previous week.
This is the first time this year that the weekly figure has exceeded 10,000 cases.
Twenty-three people died, 4,442 people were hospitalized and 8,068 showed mild symptoms.
Nearly 60 percent of the patients, or 7,279, were aged 65 or older.
Among the 47 prefectures, Tokyo reported the largest number of patients with 1,294, followed by Osaka with 952 and Aichi at 841.
Officials say the heatstroke cases are expected to peak in August and call on people to take precautions such as using air conditioners appropriately and drinking enough water.
Data from Japan’s Fire and Disaster Management Agency show 7,235 heatstroke patients were taken to hospital nationwide in June.
The figure is the second largest for the month of June since the statistics began in 2010, following 15,969 recorded in June of last year.
Two people died, 2,279 required hospitalization and 4,911 were in fair condition.
Among the total for the last month, more than half of the patients were aged 65 or older.
The largest number of patients suffered heatstroke inside houses, numbering 2,567, followed by on road, at 1,328. The next most numerous cases were from outdoor locations such as sports grounds and parking lots, totaling 953.
The fire department calls on people to take thorough precautions against heatstroke by using air conditioners appropriately and drinking enough water.
In early June and the middle of the month, high pressure systems covered eastern and western Japan, pushing up temperatures to 30 degrees Celsius or more across Japan. Daytime highs even topped 35 degrees in eastern Japan in late June.