PURPOSE: To prevent the revolution speed set from being sharply varied up and down in reciprocation by calculating the difference between the detected speed and the set speed in each prescribed time and transmitting instruction to gradually bring the engine speed close to the revolution speed set in tepwise.

CONSTITUTION: The revolution speed detector 1 of an engine transmits a pulse signal in each revolution of a revolution shaft 3 through the induction between the magnet 7 of a wheel 5 and an induction coil 9. A controller 11 receives a pulse signal from the detector 1 and calculates the revolution speed of the engine, and transmits instruction in each prescribed time, in accordance with the difference from the revolution speed set separately, and further transmits instruction in accordance with the amount of increase or reduction of the revolution speed detected in each prescribed time. In the stepping motor 17 in an adjuster 15, a slider 21 extends or contracts in accordance with the number of positive or reverse pulse signals transmitted from the controller 11, and then a throttle valve 27 is turned through a rod 23 and an arm 25, and thus the opened degree of the passage 2 in a fuel mixing apparatus 28 is adjusted.