Conventionally, Patent Document 1 (JP-2001-1749A corresponding to US 2003/0182955A1) describes regarding a vehicle air conditioner, which performs a heating of a vehicle compartment by heating air to be blown into the vehicle compartment by using both an interior heat exchanger of a heat pump cycle and a heater core.  Here, the interior heat exchanger is operated as a condenser for cooling and condensing refrigerant in the heat pump cycle, and the heat core is a heating heat exchanger for heating air by using engine coolant as its heat source. 
Furthermore, in the vehicle air conditioner of the Patent Document 1, the interior heat exchanger is located upstream of the heater core in a flow direction of air so that air having been heated by the interior heat exchanger is re-heated in the heater core.  Furthermore, a flow amount of the engine coolant supplied to the heater core is adjusted by a flow adjustment valve, and thereby the temperature of air to be blown into the vehicle compartment can be adjusted. 
In the vehicle air conditioner of the Patent Document 1, when the air flowing from the interior heat exchanger is re-heated in the heater core by using exhaust heat from a vehicle engine as its heating source, a using degree of the exhaust heat of the engine is changed so that the temperature of air to be blown into the vehicle compartment is adjusted, thereby performing a suitable heating. 
In the vehicle air conditioner of the Patent Document 1, it is necessary to make the temperature of engine coolant higher than the temperature of air heated by the interior heat exchanger, in order to suitably perform the heating of the vehicle compartment.  Thus, when the air conditioner of the patent Document 1 is used for a hybrid vehicle driven by an engine and an electrical motor for traveling, it is difficult to suitably perform the heating of the vehicle compartment by using the exhaust heat of the engine in accordance with a vehicle traveling state. 
In the hybrid vehicle, the vehicle is sometime traveled by only using a driving force of the electrical motor for traveling so as to improve fuel consumption of the engine while the engine is in a stop state.  In this state, it is impossible to sufficiently obtain exhaust heat from the engine.  As a result, the temperature of the engine coolant may be lower than the temperature of air flowing from the interior heat exchanger, and it is difficult to suitably perform the heating of the vehicle compartment. 
To overcome the above problem, the engine may be operated so that a necessary exhaust heat required for the suitable heating can be obtained, when the heating of the vehicle compartment is performed in a hybrid vehicle.  However, in this case, fuel consumption is remarkably deteriorated in the hybrid vehicle. 
This problem is also caused when the air conditioner of the Patent Document 1 is used for a fuel cell vehicle, in which an electrical power is supplied to an electrical motor for traveling from a fuel cell, and a cell coolant is used as a heat source of the heater core.  In the fuel cell vehicle, the temperature of the fuel cell itself is maintained to a predetermined temperature in order to improve power generation efficiency of the fuel cell, by controlling a flow amount of cell coolant in accordance with a vehicle traveling state. 
Thus, if the temperature of the cell coolant is set lower than the temperature of air heated by the interior heat exchanger, it will be difficult to suitably perform the heating in the vehicle compartment.  On the other hand, if the temperature of the cell coolant is increased in order to realize the suitable heating of the vehicle compartment, the power generation efficiency of the fuel cell will be decreased, thereby deteriorating the fuel consumption in the fuel cell vehicle.