PURPOSE: To detect the operated condition of the variable capacity type compressor by providing a pressure detecting means for detecting the pressure of the compressed fluid and an operated condition detecting means for detecting the operated condition of the compressor on the basis of a variation of that pressure detecting signal.

CONSTITUTION: A pressure detecting unit 20 detects the pressure of the guiding compressed refrigerant as the refrigerant pressure with a detecting element 21 in co-operation of an oscillating unit 22 and a switching circuit 23. This detected refrigerant pressure is generated as a pulsating voltage Vp through a filter 24 and a buffer 25. A computing circuit 90 computes the effective value of the pulsating voltage Vp from the pressure detecting unit 20 to generate it as the pressure signal of the compressed refrigerant in a discharge chamber. The computing circuit 90 also computes the rotating number of a compressor 10 in response to the number and the cycle of each pulsating voltage Vn from a monostable circuit 60 to generate it as the rotating number signal. The computing circuit 90 also computes the capacity of the compressor which is in proportion to the amplified voltage Vda from an amplifying unit 80 to generate it as the capacity signal.