A fuel tank system for a vehicle having an internal combustion engine, the fuel tank system comprising: a fuel storage unit having a sealing valve and configured to seal a fuel tank that stores fuel; a processing unit configured to process fuel evaporative gas in the fuel tank; and a control unit configured to diagnose a failure of the fuel storage unit and the processing unit, wherein the processing unit includes a communication passage through which the sealing valve and an intake passage of the internal combustion engine are communicated with each other, a first on-off valve configured to be opened and closed between the intake passage and the communication passage, a canister connected to the communication passage between the sealing 15 valve and the first on-off valve and configured to adsorb fuel evaporative gas in the fuel tank, a second on-off valve configured to be opened and closed between the canister and the communication passage, and a pressure generation unit connected to the canister and configured to generate pressure, and wherein the control unit is configured to perform a first failure diagnosis of diagnosing a failure of the fuel storage unit in a state where the sealing valve is closed, when the fuel storage unit is diagnosed to be normal by the first failure diagnosis, perform a second failure diagnosis of diagnosing a failure of the first on-off valve and the second on-off valve by causing the pressure generation unit to generate pressure in a state where the sealing valve is closed, and when it is diagnosed by the second failure diagnosis that there is a possibility of closed-sticking in at least one of the first on-off valve and the second on-off valve, perform a third failure diagnosis of opening the sealing valve to specify a failure as any one of closed-sticking of the first on-off valve and closed-sticking of the second on-off valve. a fuel storage unit having a sealing valve and configured to seal a fuel tank that stores fuel; a processing unit configured to process fuel evaporative gas in the fuel tank; and a control unit configured to diagnose a failure of the fuel storage unit and the processing unit, wherein the processing unit includes a communication passage through which the sealing valve and an intake passage of the internal combustion engine are communicated with each other, a first on-off valve configured to be opened and closed between the intake passage and the communication passage, a canister connected to the communication passage between the sealing 15 valve and the first on-off valve and configured to adsorb fuel evaporative gas in the fuel tank, a second on-off valve configured to be opened and closed between the canister and the communication passage, and a pressure generation unit connected to the canister and configured to generate pressure, and a communication passage through which the sealing valve and an intake passage of the internal combustion engine are communicated with each other, a first on-off valve configured to be opened and closed between the intake passage and the communication passage, a canister connected to the communication passage between the sealing 15 valve and the first on-off valve and configured to adsorb fuel evaporative gas in the fuel tank, a second on-off valve configured to be opened and closed between the canister and the communication passage, and a pressure generation unit connected to the canister and configured to generate pressure, and wherein the control unit is configured to perform a first failure diagnosis of diagnosing a failure of the fuel storage unit in a state where the sealing valve is closed, when the fuel storage unit is diagnosed to be normal by the first failure diagnosis, perform a second failure diagnosis of diagnosing a failure of the first on-off valve and the second on-off valve by causing the pressure generation unit to generate pressure in a state where the sealing valve is closed, and when it is diagnosed by the second failure diagnosis that there is a possibility of closed-sticking in at least one of the first on-off valve and the second on-off valve, perform a third failure diagnosis of opening the sealing valve to specify a failure as any one of closed-sticking of the first on-off valve and closed-sticking of the second on-off valve.