A diagnostic apparatus for an evaporative fuel processing system includes a fuel tank retaining fuel to be fed to a pressure-charger-equipped engine, a canister communicating with the fuel tank and adsorbing evaporative fuel generated therein, an upstream purge line allowing the canister and an engine intake system to communicate at an upstream side of the pressure charger, an upstream purge valve that opens and closes the upstream purge line, a pressure detector detecting pressure in the upstream purge line; a valve controller that opens and closes the upstream purge valve during pressure-charging and non-pressure-charging, respectively, a first timekeeper measuring an accumulative time in which a diagnosis execution condition is satisfied after a purge-flow diagnosis is started, and a determiner determining that the system operates normally or abnormally depending on whether the pressure decreases or not by a predetermined pressure or more from the start of the diagnosis.