Abstract: An engine cooling system includes a coolant circuit, a multi-way valve, a relief route and a relief valve. The coolant circuit includes a first route and a second route into which the coolant circuit is branched off at a branched position. The first route passes through a radiator. The multi-way valve is provided at the branched position. The relief route sets a relief source to a portion downstream of a pump and upstream of the multi-way valve in the coolant circuit, sets a relief destination to a portion downstream of the radiator in the first route, and causes coolant to flow from the relief source to the relief destination so as to bypass the multi-way valve. The relief valve interrupts circulation of coolant through the relief route when the relief valve is closed, and permits circulation of coolant through the relief route when the relief valve is open.