RELAY DEVICE AND EXTERNAL DEVICE

The relay device transmits data for writing or updating a program received from the external communication bus to an in-vehicle communication bus, which is the transfer destination, in order of reception of the data. A storage area for temporary storage can be smaller and costs can be reduced, as compared with a case where the data is stored until all data is prepared for each of in-vehicle devices. Further, the required time can be reduced as compared with a case where all data is received for each of the in-vehicle devices and then transmitted to the in-vehicle devices. The external device transmits data addressed to the in-vehicle devices to the relay device in parallel.