Vehicle network system, a gateway device and a fraud detection method

The fraud detection method used in the in-vehicle network system comprising a fraud detection ECU connected a plurality of electronic control unit (ECU) and to a bus for transferring messages on the bus, the message fraud detection ECU is transmitted on the bus a memory for storing rule information indicating a rule for transmission, fraud detection messages sent ECU using rule information by on the bus to determine whether fraud or illegal when was invalid It sends an error message including the message identifier of the message, acquires the update rule information sent from an external server by the fraud detection ECU, updates the rule information by using the update rule information.