This study was to provide the information for optimum utilization of the air-conditioning system in the human health and energy saving perspective. Subjects were 17 male and female college students(7 males and 10 females) with normal weight. They wore a short sleeved shirt, knee length trousers, socks, and underwear(0.4clo). They were asked to choose the preferred temperature from different environmental temperatures($28^{circ}C$, $25^{circ}C$). The physiological responses were measured and the subjective sensation was voted during the step changes of environmental temperature, starting at $28^{circ}C$ to $25^{circ}C$ with $1^{circ}C$ decrease every 20 minutes. The preferred temperature was $25.9{pm}0.4^{circ}C$ for males and $26.9{pm}0.2^{circ}C$ for females at $28^{circ}C$ and $24.8{pm}0.6^{circ}C$ for males and $25.6{pm}0.1^{circ}C$ for females at $25^{circ}C$. The preferred temperature decreased about $1.3^{circ}C$ while the environmental temperature changed $3^{circ}C$. During the environmental step changes, mean skin temperature decreased more in females while the oxygen uptake and rectal temperature were kept constant for both males and females. We found the preferred temperature was affected by the exposed temperature and the thermal sensation in the condition. Subjects preferred a lower environmental temperature when they were exposed to a lower temperature with cooler sensation. Therefore, in the perspective of human health and energy saving, it is recommended to start setting the air-conditioning temperature higher than the preferred temperature.