An abnormality diagnosing apparatus configured to diagnose abnormality of a fuel injection amount of a diesel engine includes: an oxidation catalyst provided in an exhaust gas passage; an exhaust gas temperature detector configured to detect an upstream-side exhaust gas temperature and a downstream-side exhaust gas temperature, the upstream-side exhaust gas temperature being an exhaust gas temperature on an upstream side of the oxidation catalyst, the downstream-side exhaust gas temperature being an exhaust gas temperature on a downstream side of the oxidation catalyst; and an abnormality determining unit configured to determine whether the fuel injection amount is excessive or not on the basis of comparison between the upstream-side exhaust gas temperature and the downstream-side exhaust gas temperature.