A data delivery system capable of reducing processing load of a server is provided. A control server includes a selecting unit selecting a TCU to which update data is to be delivered based on priority levels set in association with TCUs which are delivery targets, a message generating unit generating and transmitting a message inquiring as to whether or not the TCU is ready to accept update data to the TCU selected by the selecting unit, and a delivery control unit receiving a response message transmitted from the TCU in response to the transmitted message, selecting a TCU to which update data is to be delivered based on the received response message and requesting a delivery server to deliver update data to the selected TCU.