PURPOSE: To detect the rotation of compressor easily by providing a magnetic pickup while facing with the rotary locus of asymmetric rotary body and detecting the magnetic flux leaked from an electromagnetic clutch through spindle to said rotary body.

CONSTITUTION: A radially projecting section 81 is provided on the outercircumference of a cam rotor 8 and a magnetic pickup 7 is provided at the position corresponding with said projection while considering the cam rotor 8 as an asymmetric rotary body. The leakage flux from an electromagnetic clutch constructed with a clutch rotor 31, an electromagnet 32 and a clutch armature plate 33 secured on the outer face of a sleeve 14 of a front end plate 11 is detected by said magnetic pickup for the purpose to detect the rotation of compressor.