To attain secrecy speech between desired conference participants, independently of the connection system of a conference by separating voice data bit areas of channel data into a conference bit area and a secrecy bit area, setting the conference voice data to the conference bit area and secrecy voice data to the secrecy bit area and conducting the secrecy speech through the use of the secrecy bit area.

In this system, a B channel of an ISDN line is used to connect a plurality of terminals to a conference by the skewer connection method, a most significant bit (B7) of 8-bit B channel data is used for a control data bit, and the remaining 7 bits (B0-B6) are used for a voice data bit area for a conference speech. The lower-order 4 bits (B0-B3) are assigned to a conference bit area, and the remaining 3 bits (B4-B6) are assigned to a secrecy bit area in advance, conference voice data are set to the conference bit area and secrecy voice data are set to the secrecy bit area.