A thermostat is provided at a connection portion between a cooling water supply passage and a bypass passage. When the temperature of the cooling water lies in a low temperature range lower than a first reference temperature, the thermostat allows introduction of the cooling water from the cooling water discharge passage into the cooling water supply passage via the bypass passage and also prevents introduction of the cooling water from the radiator into the engine via the cooling water supply passage. When the temperature of the cooling water lies in a high temperature range equal to or higher than a second reference temperature which is higher than the first reference temperature, the thermostat allows introduction of the cooling water from the radiator into the engine via the cooling water supply passage and also prevents introduction of the cooling water from the cooling water discharge passage into the cooling water supply passage via the bypass passage.