A method for use in a network communication system including a plurality of electronic controllers that communicate with each other via a bus in accordance with a Controller Area Network (CAN) protocol determines whether or not content of a predetermined field in a frame which has started to be transmitted meets a predetermined condition indicating fraud. In a case where the content of the predetermined field meets the predetermined condition, an error frame is transmitted before an end of the frame is transmitted. A number of times the error frame is transmitted is recorded for each identifier (ID) represented by content of an ID field included in a plurality of frames which has been transmitted. A malicious electronic controller is determined in accordance with the number of times recorded for each ID.