Abstract

First and second air-fuel ratio sensors are disposed upstream and downstream of a catalytic converter respectively, and these sensors provide detection signals used to set first and second air-fuel ratio correction quantities, respectively, whereby an averaged air-fuel ratio corection quantity is calculated. During a steady operation in which a change in the averaged air-fuel correction quantity is below a predetermined value, a final air-fuel ratio correction quantity is calculated according to the first and second air-fuel ratio correction quantities. During a transient operation in which a change in the averaged first air-fuel ratio correction quantity exceeds the predetermined value, the second air-fuel ratio correction quantity is fixed from when the change exceeds the predetermined value until a predetermined time has elapsed after the change drops below the predetermined value. This prevents a deviation of the air-fuel ratio due to the second air-fuel ratio correction quantity during the transient operation, and maintains a good air-fuel ratio not only during the steady operation but also during the transient operation.