PURPOSE: To enable to perform the control in an air-conditioning system and the like stably as well as positively by constructing the title device such as to judge the compression capacity on the basis of the pressure detecting signals form a pressure detecting means.

CONSTITUTION: The discharge capacity of cooling medium of a compressor 10 varies according to the change of the inclination of a cam plate 23 against a driving shaft 15. The inclination of the cam plate 23 is determined by the cooling medium pressure in a cylinder 16 and a pressure chamber 17; that is, if the pressure in the pressure chamber 17 rises to be high, the pressure acting on the rear face of a piston 27 also becomes high, and the piston 27 does not thereby recede largely from the top dead center. Consequently, the position of the bottom dead center of the piston 27 changes corresponding to the pressure in the pressure chamber 17, and the inclination of the cam plate 23 coupled with the piston 27 against the right-angled face of the driving shaft becomes smaller as the pressure in the pressure chamber 17 rises, as well as the reciprocating strokes become small.