An air-fuel ratio control device for an internal combustion engine can detect the deteriorated purification state of a catalyst without delay after completion of rich or lean control, and can rapidly restore the purification state. A filter process constant for a downstream O.sub.2 sensor output is set smaller when rich or lean control are performed, thus allowing detection of the deteriorated purification state of the catalyst without delay. After the rich or lean control is completed, a dual O.sub.2 control system is used to rapidly restore the purification state. Further, the impact of downstream O.sub.2 sensor output variation when the catalyst is deteriorated can be suppressed, and stability of the dual O.sub.2 control system can be maintained.