the vacuum chamber 19 is provided with a vent hole 20 and a vacuum inlet port 22 which is connected to an intake manifold 21 by way of a conduit 21a the vacuum motor 8 further comprises a valve lever 15 which is journalled within the vacuum chamber 19 and provided thereon with a pair of valve bodies 15a and 15b to selectively close the vent hole 20 and the inlet port 22 in this vacuum motor 8 when the electomagnetic actuator 9 is energized by an output signal from the differential amplifier 7 the valve lever 15 is moved down to close the vent hole 20 by the valve body 15a and when the actuator 9 is deenergized the valve lever 15 is moved up to close the vacuum inlet port 22 thus when the vent hole 20 is closed the vacuum motor 8 moves in accordance with the pressure difference between the air chamber 17 and the vacuum chamber 19 so as to increase the degree of opening of the throttle valve 14 and when the vacuum inlet port 22 is closed the vacuum motor 8 is returned to the original position so as to decrease the degree of opening of the throttle valve 14 fig2 discloses the electronic speed control circuit discussed with reference to the block diagram of.