A data communication device includes a storage unit configured to store transmission data in a transmission ring buffer area; a read-out processing unit configured to read out the transmission data from the storage unit; and a transmission and reception processing unit configured to transmit the read-out transmission data to an external network and receive a reception confirming notification for the transmitted transmission data. The transmission and reception processing unit is configured to control the read-out processing unit so that the transmission data to be transmitted is read out based on a storage position of last data of the transmission data stored in the transmission ring buffer area and a storage position of last data of the transmitted transmission data indicated by the reception confirming notification.