According to the present invention, when an air-conditioning clutch (27) of a compressor is set on, even if an improvement of a heating capacity by a viscous heater (4) is required, a viscous clutch (7) is turned off not to transmit a rotational driving force of an engine (E) to a rotor (53) of the viscous heater, and further the rotational driving force of the engine is transmitted to the compressor. In this way, a torque applied to the engine E and a V-belt (6) can be reduced, so that a fuel consumption ratio of the engine lowers and a fuel economical performance improves. Further, because an operation of the compressor is intermitted, it is possible to remove a fog of a front windshield, so that a safety of the vehicle travelling can be prevented from being deteriorated.