Abstract: A protecting device for a car air conditioner includes a timer for timing a period of the rise in the level of a rotation signal delivered from a rotation sensor for detecting a rotational speed of a compressor with the signal being distinguished from noise. On the basis of the obtained period, a judgment is made as to whether rotation of the compressor is in an abnormal state. An electromagnetic clutch between the compressor and a car engine is controlled on the basis of the judgment. The protecting device is mounted on a housing of the compressor with the rotation sensor so that wiring is executed only in an engine room. The protecting device includes a state holding device for memorizing the abnormal state of the rotation of the compressor. The air conditioner can restart on condition that a conscious operation is performed to reset the state holding device.