A misuse detection electronic control unit in a vehicle network system including a plurality of electronic control units that communicate with one another through buses in accordance with a CAN protocol includes a transceiver unit that performs a reception step of receiving a target data frame and a reference data frame transmitted through the buses, wherein the target data frame is a data frame having a first identifier and wherein the reference data frame is a data frame having a second identifier different from the first identifier and a misuse detection process unit that performs a detection step of performing, as misuse detection for the target data frame, evaluation in accordance with a reception timing of the reference data frame and a reception timing of the target data frame on the basis of a certain rule specifying a reception interval between the reference data frame and the target data frame.