The agency said that 400 had severe symptoms that necessitated at least three weeks of treatment.
Twenty-three people died among some 13,000 who were rushed to hospitals as a result of heat-related illnesses across Japan, according to a statement made Wednesday by Japan's Fire and Disaster Management Agency (FDMA).
In the midst of an extreme heatwave spreading across the east-Asian island country, the agency's data showed that the number of people taken to hospital had surpassed 10,000 for the second consecutive week, although the figure decreased from the almost 18,400 registered the previous week.
Of those hospitalized since Aug. 5, the agency said that 400 had severe symptoms that necessitated at least three weeks of treatment, while shorter hospital stays were required for around 4,500 patients, the agency added.
Of the total people admitted for hospital treatment, around 55 percent were older than 65.
Tokyo saw the highest number of people treated for heat-related conditions at 1,465, followed by the neighboring prefecture of Saitama with 977 patients. Osaka, came in third as it saw 897 people rushed to hospitals.
According to the Japan Meteorological Agency (JMA), the highest temperature recorded in the country so far this year was logged in Joetsu City, Niigata Prefecture on the western coast, where it topped 40.3 degrees.
The heatwave is being caused by warm air from Typhoon Krosa, the tenth of the season, which may approach the western regions of Kyushu and Shikoku. Weather experts expect it to cut a path through western Japan Thursday, sweeping across the archipelago and causing unsettled fronts particularly in the regions facing the Sea of Japan.