A fault diagnosis method of an evaporated fuel processing device, wherein: - the evaporated fuel processing device comprises: - a canister including a charge port connected to an upper space of a fuel tank, - a purge passage connecting a purge port of the canister and an intake system of an internal combustion engine, - a purge control valve, - a drain passage which is configured to open a drain port of the canister to an atmosphere, and which includes a drain cut valve, and - a pressure sensor configured to sense, as a purge passage pressure, a pressure between the purge control valve and the canister in the purge passage, and - the fault diagnosis method comprises: - performing a purge by opening the purge control valve in a state in which the drain cut valve is controlled to be opened; - closing the purge valve, and estimating the purge passage pressure; - measuring an actual pressure variation by the pressure sensor; - setting a pressure threshold value; the fault diagnosis method being characterized in that it further comprises - diagnosing a closing fixation of the drain cut valve by comparing a time period necessary for the estimated pressure reaching the pressure threshold value, and a time period necessary for the pressure measured by the pressure sensor reaching the pressure threshold value. - the evaporated fuel processing device comprises: - a canister including a charge port connected to an upper space of a fuel tank, - a purge passage connecting a purge port of the canister and an intake system of an internal combustion engine, - a purge control valve, - a drain passage which is configured to open a drain port of the canister to an atmosphere, and which includes a drain cut valve, and - a pressure sensor configured to sense, as a purge passage pressure, a pressure between the purge control valve and the canister in the purge passage, and - a canister including a charge port connected to an upper space of a fuel tank, - a purge passage connecting a purge port of the canister and an intake system of an internal combustion engine, - a purge control valve, - a drain passage which is configured to open a drain port of the canister to an atmosphere, and which includes a drain cut valve, and - a pressure sensor configured to sense, as a purge passage pressure, a pressure between the purge control valve and the canister in the purge passage, and - the fault diagnosis method comprises: - performing a purge by opening the purge control valve in a state in which the drain cut valve is controlled to be opened; - closing the purge valve, and estimating the purge passage pressure; - measuring an actual pressure variation by the pressure sensor; - setting a pressure threshold value; - performing a purge by opening the purge control valve in a state in which the drain cut valve is controlled to be opened; - closing the purge valve, and estimating the purge passage pressure; - measuring an actual pressure variation by the pressure sensor; - setting a pressure threshold value; - diagnosing a closing fixation of the drain cut valve by comparing a time period necessary for the estimated pressure reaching the pressure threshold value, and a time period necessary for the pressure measured by the pressure sensor reaching the pressure threshold value.