UNAUTHORIZED CONTROL SUPPRESSION METHOD, UNAUTHORIZED CONTROL SUPPRESSION DEVICE, AND ONBOARD NETWORK SYSTEM

An unauthorized control suppression method in a network system including a plurality of electronic control units that exchange, via a communication channel, a plurality of frames, including control frames that instruct predetermined control to an object of control, includes a reception step of sequentially receiving a plurality of frames from the communication channel, and a determining step of determining whether or not the predetermined control based on the control frame received in the reception step should be suppressed, based on a set of frames received in the reception step, within a predetermined period preceding a time of reception of the control frame.