A data transmitting apparatus makes it possible to send out a retransmitting packet at early timing even when a delivery tree structure is unclear. A transmitting terminal device including this data transmitting apparatus is includes a retransmitting request receiver that receives a data retransmitting request, a receiving terminal device relevancy updater that stores a past history of the number of reception of a retransmitting request transmitted from a first data receiving device and a past history of the number of reception of a retransmitting request sent from the other data receiving devices, wherein the subject of the retransmitting request sent by the other data receiving devices is the same as the object of the retransmitting request sent by the first data receiving device, and a retransmitting destination selector that estimates a delivery tree from the history stored by the receiving terminal device relevancy updater when the retransmitting destination selector receives the retransmitting request from the first data receiving device and selects, another data receiving device positioned downstream of the first data receiving device in the estimated delivery tree.