PURPOSE: To eliminate the unnecessary disconnection processing for communication using a channel other than a designated channel by designating and registering one of two B channels for information transfer and exclusively using this B channel for each terminal.

CONSTITUTION: A channel discriminating part 21 designates and registers one of two B channels for information transfer of a bus wiring 13 as a private channel for each of terminals 14A and 14B, and communication control of a communication control part 15 is permitted only for communication using this one B channel. Consequently, terminals 14A and 14B do not communicate by B channels other than designated channels. For example, when the B1 channel is designated in the call set request from an ISDN 11, the call set request is transmitted to terminals 14A and 14B, but the terminal 14B designated and registered to B2 channel ignores this call set request. Therefore, calling and response are returned from only the terminal 14A to the ISDN 11, and response confirmation from the ISDN 11 is received to set the B1 channel of the line.