An unauthorized control suppression method for use in a network system is provided. The network system includes a plurality of electronic controllers that exchange, via a communication channel, a plurality of frames The plurality of frames includes at least one control frame that instructs predetermined control to an object of control. The method receives, sequentially, the plurality of frames from the communication channel, and determines whether the predetermined control, instructed by the control frame received in the receiving, is to be suppressed, based on a set of frames received in the receiving. The set of frames is received in the receiving within a predetermined period preceding a time of reception of the control frame.