To establish a clock subordination between CBR terminals which receive CBR service ill point-to-multipoint manner.

An ATM-multiplexing device 100a extracts a clock from a CBR data received from a CBR terminal 401, and generates an ATM cell based on the clock. Then it sends the ATM cell to an ATM network 300 via a relay line 101. ATM multiplexing devices 100b and 100c store the ATM cells, received via relay lines 102 and 103 in a cell buffer, and convert the ATM cells into non-cell type before assembling to a frame, which is stored in a frame buffer. Adaptive control parts 32b and 32c generate a clock which is adjusted and controlled for frequency, using a remaining amount of ATM cells which remain in the cell buffer and that of frames remaining in the frame buffer. The CBR data containing the clock is sent to the CBR terminals 401 and 402.