Deterioration detection system for exhaust purification apparatus

A deterioration detection system for an exhaust gas purification apparatus, comprising:

• 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 in order to add a reducing agent constituted by an ammonia precursor to exhaust gas;
• a NOx sensor disposed in the exhaust passage downstream of the selective reduction type catalyst in order to measure an amount of nitrogen oxide contained in the exhaust gas;
• calculating unit for calculating a NOx purification ratio, which is a ratio of an amount of nitrogen oxide purified by the selective reduction type catalyst relative to an amount of nitrogen oxide flowing into the selective reduction type catalyst, using a measurement value of the NOx sensor as a parameter;
• modifying unit for executing modification processing, in which the reducing agent adding valve is controlled in order to modify an addition interval thereof while keeping an addition amount per fixed time period constant, during a reducing agent addition period of the reducing agent adding valve;
• and
• determining unit for executing determination processing, in which a determination is made as to whether or not the selective reduction type catalyst has deteriorated, on the basis of a difference in the NOx purification ratio calculated by the calculating unit before and after the addition interval is modified by the modifying unit.