A fuel vapor treatment system comprising: a fuel tank for storing fuel for an internal combustion engine; a canister for adsorbing fuel vapor generated in the fuel tank; a vapor passage that provides communication between the fuel tank and the canister; a purge passage that provides communication between the canister and an intake passage of the internal combustion engine; an isolation valve for opening and closing the vapor passage; a purge valve for adjusting a flow rate of fuel vapor flowing through the purge passage; an outside air introduction passage for introducing outside air into the canister; a tank pressure sensor for detecting pressure in the fuel tank; a canister pressure sensor for detecting pressure in the canister; and a control unit that is configured to perform: a purging process, in which the control unit opens the purge valve while the internal combustion engine is running to introduce the fuel vapor adsorbed by the canister into the intake passage; and a diagnosis process, in which the control unit identifies clogging of the purge passage based on a decrease amount of the pressure in the canister that results from execution of the purging process, the fuel vapor treatment system being characterized in that the control unit is configured to further perform: an isolation valve operation process, in which the control unit opens the isolation valve when the pressure in the fuel tank is less than or equal to a first pressure, which is less than an atmospheric pressure, and the control unit closes 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; and a purging disabling process, in which the control unit disables the purging process at least in a period from when the control unit closes the isolation valve in the isolation valve operation process to when a prescribed time has elapsed from the closing of the isolation valve. a fuel tank for storing fuel for an internal combustion engine; a canister for adsorbing fuel vapor generated in the fuel tank; a vapor passage that provides communication between the fuel tank and the canister; a purge passage that provides communication between the canister and an intake passage of the internal combustion engine; an isolation valve for opening and closing the vapor passage; a purge valve for adjusting a flow rate of fuel vapor flowing through the purge passage; an outside air introduction passage for introducing outside air into the canister; a tank pressure sensor for detecting pressure in the fuel tank; a canister pressure sensor for detecting pressure in the canister; and a control unit that is configured to perform: a purging process, in which the control unit opens the purge valve while the internal combustion engine is running to introduce the fuel vapor adsorbed by the canister into the intake passage; and a diagnosis process, in which the control unit identifies clogging of the purge passage based on a decrease amount of the pressure in the canister that results from execution of the purging process, a purging process, in which the control unit opens the purge valve while the internal combustion engine is running to introduce the fuel vapor adsorbed by the canister into the intake passage; and a diagnosis process, in which the control unit identifies clogging of the purge passage based on a decrease amount of the pressure in the canister that results from execution of the purging process, the fuel vapor treatment system being characterized in that the control unit is configured to further perform: an isolation valve operation process, in which the control unit opens the isolation valve when the pressure in the fuel tank is less than or equal to a first pressure, which is less than an atmospheric pressure, and the control unit closes 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; and a purging disabling process, in which the control unit disables the purging process at least in a period from when the control unit closes the isolation valve in the isolation valve operation process to when a prescribed time has elapsed from the closing of the isolation valve. an isolation valve operation process, in which the control unit opens the isolation valve when the pressure in the fuel tank is less than or equal to a first pressure, which is less than an atmospheric pressure, and the control unit closes 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; and a purging disabling process, in which the control unit disables the purging process at least in a period from when the control unit closes the isolation valve in the isolation valve operation process to when a prescribed time has elapsed from the closing of the isolation valve.