A device for determining deterioration of a catalyst, said device comprising: an NO x selective reduction catalyst which is disposed at an exhaust passage of an internal combustion engine and selectively reduces NO x with a reducing agent; a catalyst which is disposed at a location upstream of said NO x selective reduction catalyst and has an oxidizing ability; a reducing agent supplying means for supplying the reducing agent to exhaust gas at a location upstream of said NO x selective reduction catalyst; an upstream detection means for detecting the NO x concentration at a location downstream of said catalyst having an oxidizing ability and upstream of said NO x selective reduction catalyst; a downstream detection means for detecting the NO x concentration in exhaust gas at a location downstream of said NO x selective reduction catalyst; and an estimation means for estimating the ratio of NO 2 in the NO x which inflows into said NO x selective reduction catalyst under the assumption that the degree of deterioration of said catalyst having an oxidizing ability is at a predetermined value; characterized by a calculation means for calculating the NO x removal rate of said NO x selective reduction catalyst a plurality of times at least until a local maximum value is obtained, which calculation is carried out based on the NO x concentrations detected by said upstream detection means and said downstream detection means when the reducing agent was supplied by said reducing agent supplying means; and a determination means for determining the degree of deterioration of said catalyst having an oxidizing ability, which means is carried out by comparison between the ratio of NO 2 estimated by said estimation means when the NO x removal rate of said NO x selective reduction catalyst reaches a local maximum value and a reference value of said ratio. an NO x selective reduction catalyst which is disposed at an exhaust passage of an internal combustion engine and selectively reduces NO x with a reducing agent; a catalyst which is disposed at a location upstream of said NO x selective reduction catalyst and has an oxidizing ability; a reducing agent supplying means for supplying the reducing agent to exhaust gas at a location upstream of said NO x selective reduction catalyst; an upstream detection means for detecting the NO x concentration at a location downstream of said catalyst having an oxidizing ability and upstream of said NO x selective reduction catalyst; a downstream detection means for detecting the NO x concentration in exhaust gas at a location downstream of said NO x selective reduction catalyst; and an estimation means for estimating the ratio of NO 2 in the NO x which inflows into said NO x selective reduction catalyst under the assumption that the degree of deterioration of said catalyst having an oxidizing ability is at a predetermined value; characterized by a calculation means for calculating the NO x removal rate of said NO x selective reduction catalyst a plurality of times at least until a local maximum value is obtained, which calculation is carried out based on the NO x concentrations detected by said upstream detection means and said downstream detection means when the reducing agent was supplied by said reducing agent supplying means; and a determination means for determining the degree of deterioration of said catalyst having an oxidizing ability, which means is carried out by comparison between the ratio of NO 2 estimated by said estimation means when the NO x removal rate of said NO x selective reduction catalyst reaches a local maximum value and a reference value of said ratio.