A catalyst is positioned in an exhaust path of an internal-combustion engine. A main air-fuel ratio sensor and sub-oxygen sensor are respectively positioned upstream and downstream of the catalyst. A main feedback operation is performed so that the output of the main air-fuel ratio sensor is fed back and reflected in the fuel injection quantity until the control A/F prevailing upstream of the catalyst coincides with the target A/F. A sub-feedback operation is performed so that the output of the sub-oxygen sensor is fed back and reflected in the fuel injection quantity until the air-fuel ratio of an exhaust gas flowing out of the catalyst agrees with a theoretical air-fuel ratio. Since a catalyst window varies in accordance with the intake air quantity, the fuel injection quantity is corrected so that the larger the intake air quantity becomes, the richer the control A/F is.