Vehicle-mounted network system, invalidity detection electronic control unit, and invalidity detection method

An invalidity detection method used in a vehicle-mounted network system provided with a plurality of electronic control units communicating via a bus includes: a detection step of detecting that a vehicle state satisfies a predetermined condition; and a switching step of switching, upon detection of the vehicle state satisfying the predetermined condition, an operation mode of an invalidity detection electronic control unit connected to the bus between a first mode for performing a first type of detection process of detecting an invalid message on the bus and a second mode for not performing the first type of detection process.