The aim of this research was to understand hot working environment at a construction site in summer and its effects on health of workers. In the subjective construction site, some measures, such as taking a break during work, setting tents and electric fans, and drinking cool water, had already been taken to reduce heat stress. Twelve male workers were examined. The WBGT outdoors during work varied from 23 to 34°C. The time-motion study revealed that one subject worked exceeding 7 hours, and that the other one had little rest time and drank little water during work. Few items of subjective symptoms increased after work compared with before work. In blood chemical data, electrolytes and blood urea nitrogen did not change. Blood sugar before work was significantly higher than before lunch and after work. Two subjects showed serum osmotic pressures increased after work. Two had the tendency to increase the blood pressure during work. The measures seemed effective, because the effects of work were not remarkable in general. However, some problems were still pointed out. Thus, stricter work control and health care for workers are necessary, such as controlling working hours strictly and monitoring the water intake during work.