In a fraud-detection method for use in an in-vehicle network system including a plurality of electronic control units (ECUs) that exchange messages on a plurality of networks, a plurality of fraud-detection ECUs each connected to a different one of the networks, and a gateway device, a fraud-detection ECU determines whether a message transmitted on a network connected to the fraud-detection ECU is malicious by using rule information stored in a memory. The gateway device receives updated rule information transmitted to a first network among the networks, selects a second network different from the first network, and transfers the updated rule information only to the second network. A fraud-detection ECU connected to the second network acquires the updated rule information and updates the rule information stored therein by using the updated rule information.