A source device 10 transmits streaming data. In a packet receiver 22 of a sync device 20, statistical data acquirer 23 calculates statistical data such as one-way delay time, packet loss ratio, reception rate and the like from the received streaming data and transmits it to the source device. The statistical information received by source device 20 is extracted by a packet receiver 15 and input to a channel band estimator 16. The channel band estimator 16 gives encoder 11 instructions to provide optimal encoding based on the transmission delay. Upon this, the transmission delay is calculated by assuming the minimum value of the communication delay from the start of transmission as zero. Control is performed by determining the delay time and packet loss ratio in data transmission and considering, based on these, the channel quality and bandwidth margin, so as to provide the optimal transmission bit rate or encoding rate.