FAULT DIAGNOSIS METHOD AND FAULT DIAGNOSIS DEVICE FOR EVAPORATED FUEL PROCESSING DEVICE

This evaporated fuel processing device includes: a canister (3) connected to a fuel tank (2); a purge passage (19) for introducing purge gas to an internal combustion engine (1); a purge control valve (23); a pressure sensor (32) that detects a purge passage pressure (P2); and a drain passage (25) having a drain cut valve (26). A stuck-closed fault diagnosis of the drain cut valve (26) is executed at a time (t2) when the purge control valve (23) is closed from an open state. An estimated purge passage pressure (P2est) is computed assuming an opening degree that is considered stuck-closed, and a time (t4) when the estimated purge passage pressure (P2est) reaches a pressure threshold value (Pth) is compared to a time (t3) when an actual purge passage pressure (P2rea) reaches the pressure threshold value (Pth).