A fuel tank system for a vehicle includes a fuel tank, a fuel supply passage, a canister, an evaporated fuel gas supply passage, a communication passage, a backflow prevention device, first and second pressure measurement devices, and a control device. The fuel tank stores fuel supplied by the fuel supply passage. The canister adsorbs evaporated fuel gas generated in the fuel tank and supplied through the evaporated fuel gas supply passage. The backflow prevention device is provided in a pipe line of the fuel supply passage, and prevents a backflow of the fuel from the fuel tank. The first and second pressure measurement devices respectively measure pressures in the fuel supply passage and the evaporated fuel gas supply passage. The control device diagnoses the communication passage as being blocked when a difference between pressure values measured by the first and second pressure measurement devices exceeds a predetermined value.