When half-duplex many-to-many communication is performed among a plurality of mobile stations having different codecs in a single group, transcoding is performed with a small number of codec circuits. A transcoder manages an SSRC of a received RTP packet, detaches voice information from a jitter buffer, which is identical to a latest SSRC, and inputs the detached voice information to a codec circuit. As the latest SSRC is switched, a voice sending station subject to transcode is switched, and mobile stations using the same codec share the codec circuit. Therefore, in a half-duplex many-to-many communication (group communication) system, called a professional mobile radio system, push-talk, or press-talk, it is not necessary to switch the transcoder until a radio station which takes a talk burst is changed during a single call (call session).