PURPOSE: To improve the stability of an engine by setting the maximum operating current and the minimum operating current of an electric air quantity controlling valve in accordance with the load of the engine, in the titled air-fuel ratio controller for controlling the air-fuel ratio on the basis of the output signal of an O2 senser.

CONSTITUTION: An air-fuel controlling air inlet port 21 is provided in a suction pipe 2, and is connected to an electric air quantity controlling valve 72 through an air inlet path 71. An exhaust manifold 41 is provided with an O2 senser 5, and the up and down movement of a valve part 722 is carried out by an output signal from the O2 senser 5 in a cyclic manner. In a control circuit 6, the maximum operating current and the minimum operating current of the electric air quantity control valve 72 is determined in accordance with the opening degree signal, and the control valve 72 does not open at an opening degree more than that in response to the maximum operating current and the minimum operating current. By this arrangement, introduction of air in an excessive quantity is prevented, and hence too much lean air-fuel ratio can be prevented.