PURPOSE: To miniaturize the whole of a compressor by inserting into a valve cylinder, a spool which opens/closes a bypass passage by making sliding motion in response to the magnitude of control pressure acting on pressure chambers.

CONSTITUTION: One side ends of bypass passages 35a, 41a are opened at specified positions on the cylindrical face of a cylinder, and opened/closed by a reciprocating movement of a piston 27a, and the other side ends of the bypass passages are opened at specified positions of the cylindrical face of valve cylinders 32, 38. When the other side ends of the bypass passages are opened by sliding motion of a spool 33 inserted into the valve cylinder in response to the magnitude of a control pressure acting on pressure chambers 36, 42 formed on the end faces, the fluid escapes from a bypass cylinder to the valve cylinder even if it is compressed inside the cylinder during a stroke where a piston close the one end of the bypass passage, and it is returned to the intake side and not pressurized. The fluid is then compressed inside the cylinder so as to be fed to discharge chambers 9, 11 in the strokes after the one end of the bypass passage is closed by the piston.