A gateway apparatus is connected to a plurality of buses. A data frame is received by using one of the plurality of buses as a reception bus. Whether the received data frame is a monitoring target data frame is determined based on monitoring target information. The monitoring target information is stored in a storage to associate the monitoring target data frame with a monitoring target transmission source bus connected to a transmission source electronic control unit. Whether the reception bus matches the monitoring target transmission source bus is determined based on the monitoring target information in response to determining that the received data frame is a monitoring target data frame. An abnormality detection on the received data frame is performed in response to determining that the reception bus matches the monitoring target transmission source bus.