Cooling water circuit system

A cooling water circuit system for cooling a water-cooled engine of a vehicle and for heating air blown into a passenger compartment of the vehicle, said cooling water circuit system comprising:

• means for forming a cooling water circuit in which cooling water for cooling the engine circulates;
• a radiator for cooling the cooling water in said cooling water circuit;
• a heater core for heating air blown into the passenger compartment by using cooling water in said cooling water circuit as a heating source;
• a first water pump mechanically driven by the engine, said first water pump being disposed in said cooling water circuit in such a manner that cooling water pumped by said first water pump flows through said radiator and said heater core in parallel;
• an electrically-driven second water pump disposed in said cooling water circuit at a position where cooling water to be supplied from said first water pump to said radiator and cooling water to be supplied from said first water pump to said heater core flow together, said second water pump having a suction side for sucking cooling water and a discharge side for discharging cooling water;
• and
• a check valve disposed in said cooling water circuit in parallel with said second water pump in such a manner that cooling water flows in one way from said suction side of said second water pump toward said discharge side of said second water pump.