A catalyst deterioration-determining device capable of properly performing deterioration determination, while compensating for differences in output characteristics of oxygen concentration parameter sensors disposed upstream and downstream of the catalyst, respectively. After exhaust gases flowing into the catalyst having an oxygen storage capability are switched from oxidation atmosphere to reduction atmosphere, first and second equivalent ratios indicative of oxygen concentrations, detected by first and second LAF sensors disposed upstream and downstream of the catalyst, are integrated to calculate respective first and second reducing agent amount integrated values. The integrated values are corrected by dividing them by respective average values of the first and second equivalent ratios calculated after the lapse of a predetermined time period, such that differences in gains of the first and second LAF sensors are compensated for. Then, the corrected first and second integrated values are compared for catalyst deterioration determination.