A data transmitting device divides application data into packets, transmits the packets, and continuously carries out packet transmission while receiving acknowledgement from a receiving device of the transmitted packets. The application data to be transmitted is stored in a data storage area of a data storage unit. The data is read from the data storage area, a sequence number is set, and the data is transmitted as the packet. If the read data is within a predetermined length from an end of the application data, a copy of the read data is saved in a temporary buffer area of the data storage unit. When data of a sequence number before that of first data of next application data is to be transmitted after transmission of the next application data is started, the data is read from the temporary buffer area of the data storage unit.