Provided is a fraud detection rule updating method enabling the updating of rules that serve as the basis for detecting malicious frames as necessary in an on-board network system. In an on-board network system equipped with multiple electronic control units (ECUs) that communicate via buses and fraud detecting ECUs that determine, based on fraud detection rules, whether messages transmitted on the buses conform to the rules, a fraud detection rule updating method is used in which delivery data including updated fraud detection rules is received from a server external to the on-board network system, and if a certain update condition is satisfied, the fraud detection rules in a fraud detecting ECU are updated to the updated fraud detection rules.