The sweaty weather has sent hundreds of people to hospitals and raised fears of an eventual power shortage, with Japan's largest nuclear power plant shut down since an earthquake last month.

At least 56 people have died this month in the heat wave, caused by high air pressure from the hot Pacific Ocean, according to a tally by public broadcaster NHK.

The mercury rose to 40.8 degrees Celsius (105.44 F) in Gifu prefecture in central Japan mid-afternoon Friday, a day after it soared to a domestic record of 40.9 C (105.62 F) there, the Japan Meteorological Agency said.

In Saitama prefecture, north of Tokyo, where the record 40.9-degree heat was also registered on Thursday, an 84-year-old woman was found dead by a visitor Friday morning at her home where she lived alone.

Fourteen people, most of them elderly, have died from heat stroke in landlocked Saitama with nine dead on Thursday alone, said Katsuya Okano, an official at the prefecture's disaster management division.

"The worst happened on Thursday because a series of hot days had weakened them physically," he said.

In Tokyo, an 81-year-old woman was also found dead by a neighbour early Friday in her apartment where the air conditioner was switched off.

A male worker in his 50s collapsed at a construction site and died of heat stroke in the western city of Kobe early Friday.

In Osaka, a 17-year-old member of a high school rugby team died Friday, nine days after he fell unconscious due to heat stroke during an afternoon training session, the prefectural education board said.

"Training at the school has been since limited to a light workout such as muscle training," said Masaya Matsuda of the board's student guidance division.