A vehicle fuel system according to the invention includes a fuel filler flap that blocks an opening of a fuel filler space to be openable and closable, a locking mechanism that regulates opening and closing operation of the fuel filler flap, and a detector that detects the opening and closing operation of the fuel filler flap. A controller is able to determine an operational state of the fuel filler flap multiple times within one driving cycle of a vehicle, which improves a frequency of diagnosing the operational state of the fuel filler flap and a stuck-closed state of the detector.