An object of the present invention is to detect deterioration of a selective reduction type catalyst early and improve a detection precision in a deterioration detection system for an exhaust gas purification apparatus including a selective reduction type catalyst disposed in an exhaust passage of an internal combustion engine, a reducing agent adding valve disposed in the exhaust passage upstream of the selective reduction type catalyst, and a NOx sensor disposed in the exhaust passage downstream of the selective reduction type catalyst. To achieve this object, in the deterioration detection system for an exhaust gas purification apparatus according to the present invention, during control for causing the reducing agent adding valve to add a reducing agent, the reducing agent adding valve is controlled in order to modify a reducing agent addition interval thereof while keeping an addition amount per fixed time period constant. Deterioration of the selective reduction type catalyst is then determined on the basis of a difference in a NOx purification ratio before and after modification of the addition interval.