An abnormality diagnosis apparatus for performing a leak check of an evaporative fuel processing system which includes a fuel tank, a canister for storing evaporative fuel generated in the fuel tank, a purging passage for connecting the canister to an intake system of an internal combustion engine, and a purge control valve provided in the purging passage. A process of reducing the pressure in the evaporative fuel processing system is executed with a limited flow rate of gases passing through the purge control valve when performing the leak check in an idling condition of the engine. The limited flow rate is set to a value equal to or smaller than a predetermined flow rate which is smaller than the maximum flow rate applied to the leak check performed in operating conditions other than the idling condition. The leak check is performed on the basis of a change in the pressure in the evaporative fuel processing system after the pressure reduction process.