A fraud detecting method for use in an in-vehicle network system including a plurality of electronic control units that communicate with each other via a network includes detecting whether a state of a vehicle satisfies a first condition or a second condition, and switching, upon detecting that the state of the vehicle satisfies the first condition or the second condition, an operation mode of a fraud-sensing electronic control unit connected to the network between a first mode in which a first type of detecting process for detecting a fraudulent message in the network is performed and a second mode in which the first type of detecting process is not performed.