to diaphragm 28 and a coil spring 31 which biases diaphragm 28 so as to cause valve body 29 to close a valve port 30 thus the flow of fuel drawn out of fuel supply port 24 via valve port 30 from a fuel inlet 32 is controlled by the vacuum pressure introduced into vacuum pressure chamber 26 the vacuum pressure controlling diaphragm valve 25 is generated by a control valve 33 on the basis of a signal from a control unit 34 valve 33 includes a pressure regulating valve 38 which regulates the vacuum pressure introduced via a duct 48 from vacuum pressure draw port 23 by means of a pair of springs 35 36 and a diaphragm 37 which partitions the interior of valve 33 into two chambers enclosing springs 35 36 and communicating with the atmosphere and vacuum pressure introduced via duct 48 respectively thereby producing a constant pressure and an electromagnetic regulator 40 which includes an electromagnetic valve 39 which opens and closes to mix the constant pressure with atmospheric pressure available via a duct 49 in order to produce a desired vacuum pressure in more detail the duty cycle of a drive pulse signal which opens and closes electromagnetic valve 39 at a constant frequency is adjusted to control the vacuum pressure applied to.