A radiator cap is connected to a circulating circuit at a connecting point located upstream of a water pump in a flow direction of coolant and that regulates a pressure in the circulating circuit to be within a predetermined pressure range that is higher than or equal to an atmospheric pressure at the connecting point. A rotary valve is disposed in the circulating circuit at upstream of the connecting point of the radiator cap in the flow direction of coolant. Accordingly, a cavitation is restricted from occurring, and the water pump can perform enough efficiency. A communication passage that has an upstream end and a downstream end connected to the circulating circuit may be disposed instead of the radiator cap. In this case, a pressure regulating valve is disposed in the communication passage.