In Japan, 50 % of heatstroke patients transported to hospital by ambulance are elderly. Most cases of heatstroke in the elderly occur at home. Accordingly, this study aimed to determine the relationship between the indoor thermal environment and body temperature as a critical cause of heatstroke in the elderly. A field survey comprising actual measurements and a questionnaire survey was conducted in summer 2015. The night-time standard new effective temperature (SET*) was found to be higher in multi-unit houses than detached houses, and was higher on the top floor than middle and lowest floors. Furthermore, the sublingual temperature increased by 0.03 °C per 1.0 °C increase in night-time SET*. These results will help proposals aimed at reducing the risk of heat stroke in the elderly.