To solve a problem of discomfort at driving again caused by a hot seat heated in a parking car by solar radiation in summer season.

A ring-shaped heat siphon 13, charging a refrigerant, the flow of which changes into gas-liquid two-phase flow when heated, is installed continuously from the sitting side 14 to the opposite side 15 of a seat. When a room temperature of a car begins to go up, the refrigerant is heated on the sitting side 14 of a seat 10, changing to gas-liquid two-phase flow, and starts to circulate toward the section installed on the opposite side 15, where the refrigerant radiates the heat, and then the circulation is continued. Thus, even if the temperature rises due to solar radiation, the inside of the car is kept constantly at an evaporation temperature of the refrigerant, which prevents the seat temperature is prevented from going up and a driver can sit comfortably on the seat.