The relationship between summer mortality and mean temperature in Japan was analyzed using monthly data from 1951 to 2020. During this period, the mortality rate decreased greatly and the elderly population has largely increased in Japan. A positive correlation between mortality rate and temperature was detected for all the analysis periods except for a highly disturbed feature before 1970, whereas the range of age-adjusted mortality variation per 1°C temperature anomaly has decreased by a factor of 5–10, indicating that the sensitivity of mortality to temperature has weakened over time. However, the mortality rate for deaths directly caused by heat decreased from the 1950s to the 1980s and then increased, showing a V-shaped change over the entire analysis period.