Out of connection passages connecting the engine cooling circuit and the intercooler cooling circuit, a coolant inflow passage is connected between downstream of a mechanical pump and also upstream of a main radiator of the engine cooling circuit, and downstream of a sub radiator and also upstream of an electric pump of the intercooler cooling circuit, and a coolant outflow passage is connected between downstream of the electric pump and also upstream of the sub radiator of the intercooler cooling circuit, and downstream of the mechanical pump and also upstream of the main radiator of the engine cooling circuit. An inter-cooling circuit valve is provided in the coolant inflow passage.