PURPOSE: To prevent deterioration of a quality of a conference call, by realizing the conference call by use of two channels for executing alternately addition and distribution.

CONSTITUTION: Terminal stations 2a∼2d control switches S1∼S4 as prescribed, therefore, a frame F1 inputs contents of a channel CHj and outputs them to each telephone set 3a∼3d, and adds outputs of the telephone sets 3a∼3d to the channel CHi. As a result, a result of addition of sound signals of the telephone sets 3a, 3b, 3c and 3d is obtained in the input side of the channel CHi of a synchronizing device 8. The contents of this frame F1 are delayed by a constant interval of time by a delaying circuit 9 as prescribed, and after that, are outputted by the timing of a frame F2. Subsequently, the addition result is outputted as the contents of the CHi from the device 8. On the other hand, the contents of the channel CHj, which are outputted from the device 8 are cleared in accordance with an operation of a switch S5. Also, in the frame F2, the operation which entirely reverses the preceding operation is executed, and the conference call is executed.