PURPOSE: To decrease variations in air-fuel ratio caused by changes in the operating condition of an engine, by providing a solenoid valve for performing feed-forward control of the air-fuel ratio in accordance with the operating condition of the engine, and by correcting the duty ratio of the solenoid valve in accordance with the condition of feed-back control.

CONSTITUTION: A deviation computing means 106 computes a deviation of control duty of a solenoid valve A which is determined by a feed-back control circuit 10 in accordance with a detection signal from an exhaust sensor 24. Further, a discriminating means 105 discriminates whether or not the operating condition continues for a period exceeding a predetermined time, in accordance with a detection signal from an operating condition detecting means 102. Further if the control duty of the valve A deviates from the neutral condition, exceeding a predetermined value under a uniform operating condition, a correcting circuit 107 corrects the control duty of a solenoid valve B which has been stored in a control duty memory means 103 in such a direction that the control duty ratio of the valve A approaches the neutral condition.