A compressor including a rotation detecting device for detecting the rotation of the compressor to stop the compressor in an emergency. The rotation detecting device comprises a fastening bolt fastening the casing sections together, a rotatable member supported by a drive shaft, and a magnetic sensor arranged outside the casing around the head of the fastening bolt. A magnetic flux is guided from a magnetic clutch to the rotatable member or provided by permanent magnets on the rotatable member. The rotatable member has a notch or a projection for producing periodically changing a magnetic flux which passes through the fastening bolt and is detected by the magnetic sensor.