An object of the present invention is to detect an abnormality in an exhaust gas purification apparatus having an NSR catalyst and an SCR catalyst disposed on a downstream side of the NSR catalyst more favorably. Air-fuel ratio reduction control is executed to reduce NOx occluded to the NSR catalyst when an integrated value of an amount of NOx flowing into the NSR catalyst reaches an upper limit value of an amount of NOx that can be occluded to the NSR catalyst in a normal condition. It is determined whether or not the exhaust gas purification apparatus is in an abnormal state on the basis of a NOx purification ratio of the exhaust gas purification apparatus calculated at this time.