PURPOSE: To improve the control resolution by arranging a bypass valve openable/closable upon exceeding-lowering of duty ratio over/below specific level through differential altitude in parallel with duty solenoid valve path for air-fuel ratio control of carburetor.

CONSTITUTION: Microcomputer 5 will control the air-fuel ratio through a duty solenoid valve 4 arranged in an air-bleed path of carburetor 1. If it will exceed over specific upper limit duty ratio or the resolution limit of valve under normal duty ratio control because of variation of air supply due to the differential altitude of engine, 2-way solenoid valve is opened to supply predetermined amount of air thus to lower the duty ratio correspondingly. Upon droppage of the duty ratio below the lower limit because of the differential altitude, 2-way solenoid valve 3 is closed to lower the duty ratio of valve 4 correspondingly.