An abnormality diagnosis apparatus includes an irradiation device to detect a resonance frequency by irradiating an electromagnetic wave to the NOx catalyst, and a controller to diagnose based on the resonance frequency whether the NOx catalyst is abnormal, wherein the NOx catalyst is arranged in such a position that an electric field strength inside the NOx catalyst at the time when the irradiation device irradiates the electromagnetic wave of the resonance frequency becomes larger in an upstream portion of the NOx catalyst, which is at the upstream side of a center of the NOx catalyst in the direction of flow of exhaust gas, than in a downstream portion of the NOx catalyst, which is at the downstream side of the center of the NOx catalyst.