PURPOSE: To reduce adverse influence of dispersion of an O2 sensor upon exhaust gas by arranging a control part to control so as to carry out increasing and decreasing correction on a skip value of an air-fuel ratio correction factor from a measured judgment value of an air-fuel ratio by a difference between a reaction delay value of the O2 sensor and a standard value of the sensor.

CONSTITUTION: A catalytic converter 24 is arranged in the middle part of an exhaust passage 6. An O2 sensor 26 is arranged in the exhaust passage 6 upstream of this catalytic converter 24. A control part 28 is arranged to control so as to increasing and decreasing correction on a skip value of an air-fuel ratio correction factor by a difference (α) between a preset standard value of the O2 sensor 26 and a reaction delay value or reaction delay time of the O2 sensor 26 or the actual number of integral times by calculating of the reaction delay value of the O2 sensor 26, for example, the reaction delay time or the actual number of integral times from a measured judgment value of an air-fuel ratio, for example, judgment reversed time of the O2 sensor or the number of integral times up to reversal of judgment. Thereby, influence of dispersion of the O2 sensor 26 upon exhaust gas is reduced.