An H. 261 encoder outputs coded video data together with a mode selection signal and a judging signal as to discarding or not of a packet. A packetizer receives the output signal from the H. 261 encoder and separately packetizes motion data and DCT data in the input coded data. A gateway judges whether packet including the DCT data is discarded or not with reference to the packet discarding judging signal and also to the bitrate of the network to which the gateway is connected. The gateway thus can effect bitrate conversion according to the bitrate of the network with a simple construction. The gateway may selectively discard high frequency component packets without discarding low frequency component packets, or it may not discard first importance component packets having greater influence on picture quality and selectively discard second importance component packets having less influence.