IN-VEHICLE NETWORK SYSTEM, ELECTRONIC CONTROL UNIT, AND IRREGULARITY DETECTION METHOD

A fraud detection method for use in an in-vehicle network system including a plurality of electronic control units that communicate with one another via a bus in accordance with the CAN protocol includes receiving a data frame sent to the bus and verifying, if a data frame that does not follow a predetermined rule regarding a transmission period is received in receiving a data frame, a specific identifier in the data frame.