A series of experiments has been performed to study the moisture transfer in the protective clothing exposed to a high short wave (solar) radiant heat flux at a normal condition of 20 °C with 40 % RH in terms of heat stress caused by accumulated sweat in underwear. To simulate a practical situation, an experimental set-up was composed with a hot plate simulating the skin, one layer underwear, one of air gap of 8 mm thick, and one protective clothing (PC) layer. Three types of the PC materials made of para-polyamide fibre were employed. The underwear was soaked completely wet with 560 g·m-2 distilled water to simulate the heavy sweating. Temperature of each layer was measured with thermocouples. Rate of mass transfer was calculated from the change of weight in a specific period. The water vapour transfer rate during exposure to the solar radiation dropped significantly down to 50 % to 70 % of the one without the radiation, since the water vapour evaporated from the underwear was transferred not only to the environment but also to the skin. The transferred mass flux from the underwear to the skin indicated substantial heat stress, which was estimated to be equivalent to about 3 to 4 K increase in the body temperature of an average-sized man.