A source apparatus (10) transmits stream data, which is received by a packet receiving part (22) of a sink apparatus (20) and then supplied to a statistical data acquiring part (23), which calculates, from the received data stream, statistical data, such as unilateral delay time, packet loss rate, reception rate and so on, which are then transmitted to the source apparatus. The statistical information, which is received by the source apparatus (10), is extracted by a packet receiving part (15) and then inputted to a line band estimating part (16), which instructs, based on a transmission delay, an encoding part (11) to perform an optimum encoding. At this moment, the transmission delay is calculated with the minimum value of the communication delay from the start of transmission being zero. Thus, the delay time and packet loss rate of the data transmission are determined, and the line quality and band margin are taken into account based on them, thereby providing an optimum transmission bit rate or encoding rate as a control result.