The record temperatures have lasted almost a week, in parts of Japan the mercury reached 38C or more.
It is believed that the heatwave will last at least until the end of the month, with temperatures rising every day.
Cities seem to be the worst affected: Tokyo officials report more than 3,000 emergency calls within 24 hours – a single daily record.
Japan's Meteorological Agency has issued advice to residents urging them to stay hydrated.
Avoiding outdoor exercise and seeking medical care as soon as symptoms of heat stroke occur are also critical.
More than 12,000 people have visited a hospital in the first two weeks of July due to heat complaints in the first two weeks of July.
In Kyoto, records were broken when the mercury was measured above 38C for seven consecutive days.
This is the longest period since records began.
A six-year-old boy is the youngest person to die as a result of the heatwave.
He took part in a school trip in boiling hot weather and died of heat stroke.
Officials have warned schools to delay outdoor activities until the heat wave subsides.
AccuWeather President and Founder Joel N. Myers said: "AccuWeather estimates the death toll from Japan heat wave is probably already in the hundreds despite the official delivery of just over two dozen, and we predict that the number will climb into the thousands before the heat wave ends, "
" The actual total number of human casualties may never be known, as histories of heat-related deaths are historically underestimated, as not all deaths are correctly attributed to heat and some are a consequence of worsening health problems. Deaths occur weeks later. 1
"Heat outbreaks and strokes, dehydration, migraines, sleep disturbances, and mood swings can be caused by dangerous heat."
"Historical data shows that more people are likely to be involved in vehicle accidents due to heat, such as decreased ability to concentrate, poor sleep quality, and Bad weather. "
Moisture Temperatures will be between 38 and 43 ° C by midday and afternoon this week.
High temperatures in Tokyo are usually between 29 ° C and 31 ° C, so this level is unprecedented.
The heat conditions make rescue operations after the recent floods in Japan dangerous for auxiliary workers.
In the tsunami less than a month ago, more than 200 people were affected by landslides and floods heavy rains killed.
The cleanup after this disaster continues, with about 4,500 people living in shelters.