Device and method for diagnosing evaporated fuel processing device

A diagnostic apparatus for an evaporated fuel processing apparatus, wherein the evaporated fuel processing apparatus includes a blocking valve configured to selectively allow and shut off communication between a canister and a fuel tank, and adsorbs evaporated fuel by the canister by opening the blocking valve during fuel filling, wherein the evaporated fuel occurs in the fuel tank, and performs a purge operation of introducing purge gas into an intake air line of an internal combustion engine with the blocking valve closed while the internal combustion engine is operating, wherein the purge gas contains a fuel component released from the canister,

• wherein the diagnostic apparatus is configured to diagnose whether or not the blocking valve is abnormal, based on sensing of evaporated fuel flowing through the blocking valve while the purge operation is being performed.