PURPOSE: To select an optional subscriber to make broadcast without revising a broadcast line and to set a sound volume of a subscriber speaker to the maximum sound volume state.

CONSTITUTION: A polarity detection section 71 is connected to talking lines 11-1n connecting individually to a station side, detects the polarity of talking lines and when the station side inverts the polarity, a switch 73 is operated to turn off the speaker. A bias detection section 72 connects to a broadcast line 21 connecting in common to the station side to detect a bias of the broadcast line and the switch 74 is operated when the station side applies a bias to set a sound volume adjustment variable resistor 77 to the maximum sound volume state.