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 Controller Area Network (CAN) protocol is provided. The method includes receiving at least one data frame sent to the bus, verifying a specific identifier in the received data frame only if the received data frame does not follow a predetermined rule regarding a transmission period and a state of a vehicle having the in-vehicle network system mounted therein is a predetermined state, detecting the received data frame as an authenticated data frame if the verification is successful, and detecting the received data frame as a fraudulent data frame if the verification fails.