We compared the difference of heatstroke risk of marathon runners in the Summer Olympics on the basis of the percentage of weight decrease due to sweating using potential effective sweating, between Tokyo and Sapporo. For the women’s marathon, the difference in the rate of decrease in body weight was 0.42% when the start time was 11:00, with 7.51% loss for Tokyo and 7.09% for Sapporo. Nevertheless, when the start time was 7:00, as planned, the difference decreased to 0.28%, with a 6.94% loss for Tokyo and 6.66% for Sapporo. The estimated thermal load on runners when starting the race at 7:00 on August 8 in Sapporo was equivalent to starting at 5:30 or at 16:00 on August 2 in Tokyo. For the men’s marathon, the start time with the maximum rate of decrease in weight was 10:00, with a 6.91% loss for Sapporo and 7.42% for Tokyo. If the race started at 7:00, the rate of decrease in body weight was 6.50% for Sapporo and 7.00% for Tokyo, a 0.50% difference. However, as a result of the analysis based on the relation between wet bulb globe temperature and M-lE _s (equivalent to the thermal load to the human body by net radiation and sensible heat exchange), severe weather conditions might place a large thermal load on runners, even if the race venue is changed to Sapporo.