PURPOSE: To avoid unexpected interference of the conference by detecting the state of use of each conference room, discriminating the propriety of extension of a current conference in response to the detected output and reservation information of a storage device so as to control a line changeover device and to send a message.

CONSTITUTION: Lines 2a, 2b of conference rooms MR 1a, 1b, for example, are connected by a line switching device SW 3 under the control of a controller CNT 4 to apply line constitution between the rooms MR 1a, 1b. When the rooms MR 1a, 1b are in conference, a detector DET 6 in this case detect power application of the rooms MR 1a, 1b when the time reaches just before the end time by reservation information of a storage device MM 5 and gives the use information to the CNT 4 as the detection output. Thus, the propriety of extension of the current conference is discriminated by the CNT 4 in response to the detection output and the succeeding reservation information of the MM 5 and when there is any margin till the start time of the succeeding reservation information, the CNT 4 controls the SW 3 to keep the current connection state and to control a message generator MSG 7 thereby sending a message representing the possible extension of the current conference.