A failure diagnosing apparatus makes an judgment as to whether or not a failure occurs in an evaporated fuel processing apparatus in which an evaporated fuel within a fuel reservoir is adsorbed by an activated charcoal within a canister and the evaporated fuel adsorbed by the activated charcoal is purged to an intake system of an internal combustion engine under a certain operational condition. An atmospheric air is introduced into the fuel reservoir when the fuel is supplied to the fuel reservoir. Accordingly, there is a fear that a misdiagnosis is performed if the failure diagnosis is performed during fuel supply. Accordingly, when the internal combustion engine is in operation and in fuel supply condition, the failure judgement process by the failure diagnosing apparatus is forbidden.