PURPOSE: To improve fuel consumption of an engine, by controlling a fuel injection amount so as to obtain the minimum output voltage of an O2 sensor at steady operation of the engine and performing the steady operation of the engine with its air-fuel ratio in a lean range in he vicinity of the misfiring limit.

CONSTITUTION: When an engine is operated under a steady condition, a detector 11 detects a fixed operating condition of the engine to generate a detection signal (e) of steady operation and open a gate circuit 12, and an output of an integrating circuit 10 is applied to a driver circuit 13 via the circuit 12, while a circuit 13 generates a driving signal, in which air-fuel ratio is controlled to a lean state in accordance with an output of the circuit 10, then a fuel injection valve 14 injects fuel of amount in accordance with the driving signal. In this way, the injection amount is gradually decrease till an O2 sensor generates a minimum output, that is, till an output of the circuit 10 becomes zero, and air-fuel ratio is controlled to a lean state to reach air-fuel ratio A or B of the misfiring limit, while the injection valve 14 is controlled to be within a preset range in a smaller side of air-fuel ratio of the misfiring limit by a feedback control characteristic in the circuit 13, then fuel consumption is improved.