A deterioration determining device for an exhaust emission reduction device which is capable of improving the accuracy of determination of deterioration of a catalyst as a target, when another catalyst is disposed upstream of the target. The deterioration determining device for determining deterioration of a downstream one of two catalysts includes an ECU. After switching the air-fuel ratio from a learn value to a rich value, the ECU calculates an upstream oxygen storage capability using respective equivalent ratios detected by two LAF sensors, calculates a correction coefficient according to the value of the capability, calculates a basic value of a downstream oxygen storage capability using equivalent ratios detected by two LAF sensors, and multiples the basic value by the correction coefficient to calculate the downstream oxygen storage capability. When this is smaller than a predetermined value, the downstream catalyst is determined to be deteriorated.