But the heat doesn't seem to have sapped business energy.
Figures out Monday (October 2) showed sentiment improving in the third quarter.
The closely watched "tankan" index of big-business mood beat forecasts.
For major manufacturers it hit 9 in September - up from 5 in June and marking a second straight quarter of improvement.
The survey showed companies also expect conditions to keep improving.
For big non-manufacturers the index hit 27 - the highest figure since late 1991.
The gains came after many firms said they were able to pass on higher costs to consumers.
Falling raw material costs also helped lift the mood.
Now the numbers are fuelling hopes that a rebound is taking hold.
One economist told Reuters that the economy was now likely to keep expanding at above-trend pace.
That could add to mounting staff shortages, and drive prices higher.
The worries for Japanese firms are now mostly overseas.
Sluggish global demand and an uncertain recovery in China are both concerns.
Even so, the robust tankan figures will be closely examined by the Bank of Japan, as it decides when or whether it is finally ready to start raising rates.
