A catalyst degradation determination device is provided that can determine the degradation of a selective reduction catalyst with high precision while also suppressing a temporary decline in purification performance. By way of controlling a urea injection device, the catalyst degradation determination device increases, in a selective reduction catalyst in a state in which the storage amount is a maximum, the storage amount thereof by a detection reduced-amount portion DSTNH3—JD and then decreases the amount until it is determined that ammonia slip has occurred. Then, degradation is determined based on the time at which the slip determination flag FNH3—SLIP was set to “1” when fluctuating the storage amount. The detection reduced-amount portion DSTNH3—JD is set to a value that is larger than the storage capacity of the selective reduction catalyst in a degraded state and smaller than the storage capacity of the selective reduction catalyst in a normal state.