A compressor drive torque estimating device suppresses a discrepancy between an estimated drive torque, which is based on a torque estimating device of a compressor, and an actual drive torque of the compressor. The discrepancy is due to delay in switching between employment of the torque estimating device and the actual drive torque of the compressor in judging the compressor torque. Thus, even in a transitional state immediately after the start of compression, the idling speed of an engine driving the compressor is controlled based on an accurate estimate of the compressor torque, to improve the stability of the engine idle speed.