PURPOSE: To enable to minimize the fluctuation of torque that is caused during the process when fluid is compressed, by constituting the curved contour of the circumferential surface of a cam for performing one cycle of operation of a compressor by the combination of six different curved lines.

CONSTITUTION: The curved contour of the circumferential surface of a cam for performing one cycle of operation of a compressor, i. e., processes of suction, compression and discharge is constituted by the combination of six elements. That is, the section of the cam surface for increasing the vane projection and the section for decreasing the vane projection are both constituted by a curve that causes increase and decrease of the vane projection according to the rate of change of a function sin θ, so as to decrease the change of speed at which the vanes are projected and retracted. Further, a first and a second steady sections are provided for keeping the projection of the vanes constant and increasing the overlap of the torque variation range of the vanes. Thus, it is enabled to minimize the torue variation as a whole.