PURPOSE: To add a sound signal from a downstream node and its own node terminal and to execute transmission to an adjacent upstream node by introducing relation, which is based on a communication tree between a master conference node, and plural slave conference nodes, between the conference nodes.

CONSTITUTION: Terminals are housed in terminal housing ports 220-222 and the other conference node is connected in conference node connecting ports 230-232. In an adder 201, the plural sound signals are added and an added sound signal is generated. A subtracter 210 is provided for each terminal and the sound signal to be supplied by the correspondent terminal is subtracted from the added sound signal and sent to the correspondent terminal. A gate 202 is equipped with plural input ports, correspondent output port and one specified output port. Then, one sound signal is selected out of input sound signals and outputted to the specified output port and the other sound signal is outputted to the output port corresponding to the input port. A switch 203 connects one input sound signal to the plural output ports.