The object of the present invention is to provide a differential pressure control valve capable of preventing refrigerant from leaking along the outer periphery of a piston for actuating a main valve. An annular diaphragm has an inner peripheral edge portion clamped between a main valve piston and a fixing ring secured to the main valve piston, and an outer peripheral edge portion clamped between a body and a cylindrical member secured to the body, and the cylindrical member slidably receives the main valve piston. Thus, the outer periphery of the main valve piston is completely sealed by the diaphragm and inlet pressure P1 is introduced from a refrigerant introduction chamber into a piston chamber only through an orifice, whereby characteristics of the differential pressure control valve can be stabilized.