A data transmission device, comprising: a receiver configured to receive encrypted data; a transmitter configured to transmit received data of the receiver; and a controller configured to control the transmission speed of the transmitter depending on an amount of received data, wherein the controller is configured to transmit initial data of the received data without limiting its transmission speed; wherein the controller is configured to determine whether to transmit the remaining data of the received data using pacing transmission, the pacing transmission limiting the transmission speed of the transmission device by adjusting the time interval between packets of the remaining data to be sequentially transmitted; wherein the controller is configured to determine whether to carry out pacing transmission with respect to the remaining data according to one of conditions (a) and (b): (a) determining not to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are currently transmitted at maximum transmission speed, except for when a pacing transmission transition condition is satisfied, wherein the pacing transmission transition condition is satisfied when an amount of received data continuously exceeds an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is above a predetermined threshold representing an amount of data subjected to the pacing transmission; and b) determining to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are not currently transmitted at the maximum transmission speed, except for when a maximum transmission transition condition is satisfied, wherein the maximum transmission condition is satisfied when an amount of continuously received data falls below an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is below the predetermined threshold. a receiver configured to receive encrypted data; a transmitter configured to transmit received data of the receiver; and a controller configured to control the transmission speed of the transmitter depending on an amount of received data, wherein the controller is configured to transmit initial data of the received data without limiting its transmission speed; wherein the controller is configured to determine whether to transmit the remaining data of the received data using pacing transmission, the pacing transmission limiting the transmission speed of the transmission device by adjusting the time interval between packets of the remaining data to be sequentially transmitted; wherein the controller is configured to determine whether to carry out pacing transmission with respect to the remaining data according to one of conditions (a) and (b): (a) determining not to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are currently transmitted at maximum transmission speed, except for when a pacing transmission transition condition is satisfied, wherein the pacing transmission transition condition is satisfied when an amount of received data continuously exceeds an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is above a predetermined threshold representing an amount of data subjected to the pacing transmission; and b) determining to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are not currently transmitted at the maximum transmission speed, except for when a maximum transmission transition condition is satisfied, wherein the maximum transmission condition is satisfied when an amount of continuously received data falls below an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is below the predetermined threshold. (a) determining not to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are currently transmitted at maximum transmission speed, except for when a pacing transmission transition condition is satisfied, wherein the pacing transmission transition condition is satisfied when an amount of received data continuously exceeds an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is above a predetermined threshold representing an amount of data subjected to the pacing transmission; and b) determining to carry out the pacing transmission with respect to the remaining data of the received data when the remaining data are not currently transmitted at the maximum transmission speed, except for when a maximum transmission transition condition is satisfied, wherein the maximum transmission condition is satisfied when an amount of continuously received data falls below an amount of data which can be transmitted at the maximum transmission speed by a predetermined amount of data or more for a predetermined time, and when the amount of the received data is below the predetermined threshold.