To obtain a control device installed near the compressor of a car air-conditioner.

At a normal time, the signal from a rotary sensor 25 is outputted to a judging circuit 22 after being outputted repeatedly below a prescribed time interval and rectified by a detection circuit 26. The judging circuit 22 is operated, while this signal is outputted and an electric power is outputted to the control object instrument of an electro-magnetic clutch through an amplifying circuit 23. When the signal interval of the sensor 25 exceeds a prescribed value by the inconvenience of the compressor and the slip of a belt, the output from the judging circuit 22 is cut off and the electric power supply to the control object instrument is intercepted. A temperature compensating circuit 28 is constituted by a part matched with the detection circuit 26 or having a similar temperature characteristic and can be used to even the part with a severe change of a circumference temperature near the compressor.