Although we can construct intercommunication environments by high quality video multicast streams such as DV (Digital Video) streams on super high speed networks, it is difficult for the reasonable network service users to join the high quality communication session. If we can use transcoding functions that include translator and mixer functions defined by RTP, it is able to transcode a media stream format into another stream format. Media transcoding functions are usefulness for intercommunication on heterogeneous computer networks. However, it is difficult for distributed communication systems to prepare the suitable transcoding nodes in advance because the required intercommunication environments are not always static. Accordingly, we should consider dynamic transcoding functions with relocatable decision methods. In this paper we propose a novel middleware system that realizes dynamic transcoding functions by extendable media stream.