This study investigated the effect of a thermal swimsuit (TSS) on children's thermal sensation and formative evaluation during elementary school swimming classes. In addition, application of the TSS in different environmental conditions in an outdoor swimming pool and children's physical characteristics were studied. The study participants were 62 sixth-grade elementary children, who were divided into two groups (experimental: n=32, control: n=30). The investigation consisted of a pre-test, and a post-test after three days of TSS intervention. A questionnaire survey concerning children's thermal sensation and formative evaluation of the class was conducted after each class. It was found that on days when the weather was cold, the experimental group significantly attenuated the cold sensation and showed higher motivation and interest in the class. Additionally, the TSS was able to decrease the number of students who could not continue swimming in the water because of coldness. These results indicated that swimming classes could be held even in a cold environment when the TSS was used. Furthermore, it was suggested that application of the TSS as a teaching aid would compensate for the thinner physique of children.