PURPOSE: To sufficiently reduce the difference in pressure between the inside and the outside of a compression chamber by providing a pressure regulating valve for constantly maintaining the pressure in the non-compression chamber in a housing at the intermediate pressure between suction pressure and discharge pressure.

CONSTITUTION: A fluid is compressed by utilizing a phenomenon in which the capacity of a compression chamber 17 formed by the inner walls of a housing 11 and a flexible member 14, which is provided in the housing 11, and which comprises a diaphragm or a bellows, varies by the reciprocation of the flexible member 14. A pressure regulating valve for constantly maintaining the pressure in a non-compression chamber 36 in the housing 11 at an intermediate pressure between a suction pressure and a discharge pressure, is provided. A suction port 32 and the first chamber of the pressure regulating valve, a discharging port 33 and the second chamber of the pressure regulating valve, and the non- compression chamber 36 and the third chamber of the pressure regulating valve are each communicated with one another by a piping 37. The pressure in the non-compression chamber 36 is constantly maintained at the intermediate pressure between the discharging pressure and the suction pressure.