interposing the circumferential fringe of the diaphragm therebetween the casing 60 is integrally provided with a plurality of stud bolts 70 for mounting the vacuum actuator to an external member the diaphragm 8 is cup shaped so as to be substantially complementary to the inner surface of the diaphragm unit casing 60 a flat middle portion 61 of this diaphragm 8 is interposed between a pair of discs 62 and 63 which are securely joined together by rivets 64 the inner disc 62 located inside the vacuum chamber 9 is substantially conformal to the flat middle portion of the diaphragm 8 while the other or the outer disc 63 is slightly greater than a central opening 65 provided in the central part of the diaphragm casing 60 a conical coil spring 7 is interposed between the inner disc 62 and the solenoid valve unit casing 80 and biases the diaphragm 8 in the direction to increase the volume of the vacuum chamber 9 the outer surface of the outer disc 63 is provided with a wire holder 10 consisting of a hollow projection which projects out of the central opening 65 of the diaphragm casing 60 this wire holder 10 is provided with a side slit 66 extending along the whole length thereof a pair of triangular reinforcement ribs 69 extending between the edges of the slide slit 66.