Provided is a relay device in which when data read from a database is transmitted to external devices, the transmission and reception of the data between the external devices is relayed. Data required by in-vehicle devices must be generated from data. The relay device does not send the data that is calculation source data for generating the data to each of the in-vehicle devices, but generates the data that is a calculation result, stores the data in the database, and sends the data to the in-vehicle devices as required. In addition, the relay device determines whether data needed to be generated is required to be regenerated or not and, only when it is determined that the already stored data is old and regeneration of the data is needed, regenerates the data using the data that is calculation source data.