To automatically identify the prescribed one of many extension telephone sets by sending the identification information on the extension telephone set that is connected to a telephone circuit via a private branch exchange following a connection controller to this controller via an identification information sending device contained in the extension telephone set.

The call signal of the originating side is sent to an extension telephone set 3 via a telephone circuit, a connection controller 1 and a private branch exchange 2. Under such conditions, the telephone circuit is tentatively connected to the controller 1 and all telephone sets 3 ring their bells. The off- hook signal of the telephone set 3 that is first hooked off sends the identification information request signals to the hooked-off telephone set 3 and an identification information sending device 6 including a closed circuit via the controller 1. Thus, the contained identification information is read out and sent to the controller 1 via a built-in identification information sending part. Based on the received identification information, the controller 1 switches its tentative connection secured to the telephone set 3 to the main connection to the telephone circuit.