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 second electronic control unit connected to the network. A first mode in which a first type of detecting process for detecting a fraudulent message in the network is performed is switched to a second mode in which the first type of detecting process is not performed upon detecting that the state of the vehicle satisfies the first condition. Moreover, the second mode is switched to the first mode upon detecting that the state of the vehicle satisfies the second condition.