An anti-fraud method for use in an in-vehicle network system including a plurality of electronic control units that exchange, in an in-vehicle network, data frames, each having added thereto a message authentication code (MAC). The method includes generating a first MAC by using a MAC key and a value of a counter that counts a number of times a data frame having added thereto a MAC is transmitted to the in-vehicle network. The method also includes performing verification that the data frame received has added thereto the generated first MAC and incrementing a number of error occurrences when the verification has failed for the data frame, the data frame including a predetermined ID. When the number of error occurrences exceeds a predetermined threshold, a process associated in advance with the predetermined ID is executed.