The ventilation efficiency of clothing was investigated by a trace gas method using a manikin wearing an impermeable overall under an isothermal condition, where the ventilation occurred only through the openings by diffusion. The ventilation patterns were different for each part of the body. The ventilation efficiency in the clothing microenvironment of the arm and the leg greatly depended on the distance from each opening when the wrist- or the ankle-opening was opened. When both side of wrist- or ankle-openings were opened to provide the opening area double respectively, the ventilation efficiency did not correspondingly increase twice, as compared one side opened. Even though it as certainly affected by the opening area, the ventilation efficiency was more significantly influenced by the position of openings.