PURPOSE: To reduce the magnitude of air-fuel ratio control set by feedback control by correcting the level of a basis air-fuel ratio according to the change in the atmospheric pressure.

CONSTITUTION: The captioned device is provided with detectors 10W21 comprising sensors for sensing throttle valve opening, suction pipe negative pressure, amount of suction air, engine temperature, revolution speed, and exhaust composition, and for sending an input signal to an electronic control device 15 while detecting the state of engine operation. And, a driving current composed of pulse waves is sent from the electronic control device 15 to a solenoid 23 which allows a control valve 22 for changing over a low speed jet 10 to be opened and closed, and driven little by little for the purpose of making feedback control. An atmpsheric pressure sensor 27 is made of a semiconductor, and said signal current is inputted to the electronic control device 15, and thereby a driving current is sent to a prime mover of a main system control valve, allowing a basic air-fuel ratio to be corrected according to the changes in the atmospheric pressure.