In an onboard gateway apparatus, when data is received from one network, the received data is stored in the buffer. The buffer includes a first storage area for storing data in the order of priority of an identifier (ID) attached to the data and a second storage area for storing data in the order in which the data is received. A control means stores the received data in one of the first and second storage areas based on the attached identifier, and sends the data to another network in accordance with the priority of the ID of the data. As to data passing through the first storage area, sending in the order of priority is ensured. As to data passing through the second storage area, sending in the order in which the data is received is ensured.