a vacuum chamber 9 in cooperation with the casing 80 of the solenoid valve unit 1 a wire holder 10 is attached to a central part of the diaphragm 8 so as to project out of the casing 60 of the diaphragm unit 2 thus dependent upon the magnitude of the negative pressure in the negative pressure chamber 9 the wire holder 10 is axially displaced and actuates a accelerator pedal not shown in the drawings by way of a control cable as described in greater detail hereinafter a first port of the vacuum valve 3 is connected to an engine intake system not shown in the drawings by way of a conduit 11 and a check valve 12 while a second port of the vacuum valve 3 is communicated with the vacuum chamber 9 within the casing 80 as described hereinafter the conduit 11 is also connected to an accumulator 13 for storing vacuum or negative pressure therein thus by opening this vacuum valve 3 the negative pressure in the negative pressure chamber 9 is increased and the diaphragm 8 is pulled inwardly against the spring force of the conical coil spring 7 thereby actuating the accelerator pedal in the direction to increase the vehicle speed.