A processor may perform a purging process, which may introduce fuel vapor adsorbed by a canister into an intake passage. The processor may identify clogging of a purge passage based on a decrease amount of the pressure in the canister that results from execution of the purging process. The processor may open an isolation valve when the pressure in a fuel tank is less than or equal to a first pressure, which is less than an atmospheric pressure, and close the isolation valve when the pressure in the fuel tank is greater than or equal to a second pressure, which is greater than the first pressure and less than the atmospheric pressure. The processor may disable the purging process at least in a period from when the processor closes the isolation valve to when a prescribed time has elapsed from the closing of the isolation valve.