A diagnostic apparatus includes a fuel tank; a canister adsorbing evaporative fuel; an upstream purge line allowing the canister and an engine intake system to communicate upstream of a 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; a timekeeper measuring an accumulative time in which a diagnosis execution condition is satisfied after a purge-flow diagnosis starts; and a determiner determining whether evaporative fuel processing system operates normally or abnormally depending on whether the pressure decreases by a predetermined pressure or more from the start of the diagnosis. Every time the upstream purge valve opens after the condition is satisfied, the timekeeper measures the accumulative time after a predetermined delay time elapses from when a valve opening command is output.