PURPOSE: To increase or decrease the number of revolutions of a room fan motor slowly so as to realize car environment which makes passengers comfortably by changing output frequencies of two inverters alternately in several step operations.

CONSTITUTION: In the first frequency process step, output frequency of a second inverter 6 is fixed, the number of revolutions of a room fan motor 5 is kept constant, output frequency of a first inverter 3 is increased, and the number of revolutions of a compressor 2 is increased. In the second frequency process step, output frequency of the first inverter 3 is fixed, the number of revolutions of the compressor 2 is kept constant, output frequency of the second inverter 6 is increased slightly, and the number of revolutions of the room fan motor 5 is increased slowly to make it move to the operating point. In the third frequency process step, output frequency of the first inverter 3 is decreased while output frequency of the second inverter 6 is fixed again in order to make car temperature come close to the target temperature.