A method for dealing with unauthorized frames that makes it possible to take appropriate measures when an unauthorized data frame is detected in a vehicle network system is provided. A plurality of ECUs in the vehicle network system are connected to a bus used for communicating frames. In the method for dealing with unauthorized frames, if a misuse detection ECU that checks a frame appearing in the bus detects an unauthorized frame that does not comply with a certain rule and a certain prevention condition is satisfied, a process for preventing the plurality of ECUs from performing a process corresponding to the unauthorized frame is performed (an error frame is transmitted) or, if the certain prevention condition is not satisfied, the process is not performed.