PURPOSE: To attain individual selection by connecting a private branch of exchange with extension circuits having the same number of slave stations so as to connect branching type exclusive lines to an optical private branch of exchange.

CONSTITUTION: In calling a slave station 7a of a branching type exclusive line 6 from an extension telephone set 4a, an extension number assigned to a slave station 7a is dialed at first from the extension telephone set 4a. The private branch of exchange 1 receives this number by an outgoing register 8 and transmits a calling signal to the corresponding extension circut 5a. A branching circuit adaptor 3 to which the extension circuit 5a is connected makes calling to the slave station 7a corresponding to the extension circuit 5a receiving this calling signal and displays the incoming call by the slave station 7a. When the branch line adaptor 3 detects the response at the slave station 7a, the branch line adaptor 3 closes the extension circuit 5a. The private branch of exchange 1 connects the extension telephone sets 4a and 5a through an extension trunk 2 as the response of the slave station depending on the closing of the extension circuit 5a and the communication is made possible.