A thermal conductive member is mounted on a heat generating object. An air cooling member is attached to the thermal conductive member for radiating heat into air. A liquid cooling member is removably attached to the thermal conductive member for absorbing heat from the thermal conductive member. The circulation pump operates to allow the flow of the coolant into the flow passage of the liquid cooling member. In this case, the thermal energy of the heat generating object is transferred to the air cooling member through the thermal conductive member. The heat radiating member serves to radiate the thermal energy into the air from a larger surface area. The heat generating object can in this manner be cooled down even without the liquid cooling member.