A data transmission method is provided, including adjusting a data transmission speed when a number of retransmission of a data transmission unit reaches a predetermined number, wherein the retransmission is performed when a transmission failure occurs, transferring the data transmission unit at the adjusted data transmission speed, and readjusting the adjusted data transmission speed to a basic data transmission speed when data transmission is successful, and reducing the basic data transmission speed when at least one of a retransmission rate and a number of the transmission failure satisfies a predetermined threshold condition.