A viscous heater (9) for heating cooling water for cooling a water cooled engine (E) is disposed in a cooling water circuit (W), and a front heater core (15) is disposed in the cooling water circuit at a downstream side of the viscous heater with reference to a flow direction of the cooling water. Only when a heating operation is the maximum heating operation or the maximum heating operation is desired, a viscous clutch (7) is turned on to transmit a driving force of the engine to the viscous heater. When the maximum heating operation is not desired, the viscous clutch is turned off so that the driving force of the engine is not transmitted to the viscous heater. Therefore, a load of the engine is reduced, and the cost for running the vehicle can be lowered.