When the temperature of coolant in an engine is greater than or equal to a half-warm-up determination value, an engine cooling control section opens a valve to mix the coolant in two coolant circuits. Accordingly, even if the temperature of the coolant in the engine fluctuates due to mixing coolants at different temperatures, such fluctuation occurs in a temperature range lower than the determination value for the warm-up completion of the engine. This prevents a control procedure for the time before the warm-up completion and a control procedure for the time after such completion from being performed in a repeating, alternating manner. As a result, when the coolant circulating in the first coolant circuit and the coolant circulating in the second coolant circuit are mixed together, control that should be performed based on the coolant temperature in the engine is carried out without hindrance.