An exchange capable of increasing the number of participants in a conference without increasing conference circuits for conference communications. The exchange (2) switches connections either between a line wire (11) and a plurality of extensions (10a) to (10c) or between plural extensions and plural line wires. The exchange (2) comprises: a memory (27) registered with plural telephone numbers grouped; and a controller (26) which is notified of a demand of the conference communications from an extension telephone (3a) connected with the extensions (10a) to (10c), for acquiring other telephone numbers of the same group of the telephone number of the extension telephone (3a) thereby to form a channel for unidirectional communications, and which is notified of a demand from the extension telephone (3a) for bidirectional communications with the telephone selected from the other telephones in the unidirectional communications, for forming a channel so that the unidirectional communications with the telephone of the selected telephone number may be the bidirectional communications.