This study was conducted to obtain basic data in improving the health of Koreans, saving energy and protecting environments. This study investigated the effects of wearing thermal underwear for keeping warm in the office in winter where temperature is not as low as affecting work efficiency, on thermoregulatory responses and subjective sensations. In order to create an environment where every subject feels the same thermal sensation, two experimental conditions were selected through preliminary experiments: wearing thermal underwear in 18°C air (18-condition) and not wearing thermal underwear in 23°C air (23-condition). Six healthy male students participated in this study as experiment subjects. Measurement items included rectal temperature (T_re), skin temperature (T_sk), clothing microclimate temperature (T_cm), thermal sensation and thermal comfort. The results are as follows: 1) T_re of all subjects was maintained constant at 37.1°C under both conditions, indicating no significant differences. 2) (Tmacr)_sk under the 18-condition and the 23-condition were 32.9°C and 33.7°C, respectively, indicating a significant level of difference (p<0.05). 3) Among local skin temperature, trunk part (forehead and abdomen) did not show significant differences. After 90-min exposure, the skin temperature of hands and feet under the 18-condition was significantly lower than that under the 23-condition (p<0.001). 4) More than 80% of all the respondents felt comfortable under both conditions. It was found (Tmacr)_sk decreased due to a drop in the skin temperature of hands and feet, and the subjects felt cooler wearing only one layer of normal thermal underwear at 18°C. Yet, the thermal comfort level, T_re and T_cm of chest part under the 18-condition were the same as those under the 23-condition. These results show that the same level of comfort, T_re and T_cm can be maintained as that of an environment about 5°C higher in the office in winter, by wearing one layer of thermal underwear. In this regard, this study suggests that lowering indoor temperature by wearing thermal underwear in winter can contribute to saving energy and improving health.