Abstract: An ATM cell multiplexer is arranged so that in the up direction from the terminal equipment to the ATM switchboard cells from a cell terminal portion in CLAD units are held in a cell holding portion through a Utopia Level 2 interface under the control of a communication controller in the CLAD units, and an ATM bus scheduler makes the cell holding portion transmit the cells to an ATM bus by assigning a transmission right for every cell holding portion of the CLAD units in accordance with a preset schedule table based on at least one of predetermined service categories and a traffic control corresponding to a traffic quantity, while in the down direction from the ATM switchboard to the terminal equipment the cells are broadcast from the ATM bus to the cell holding portion of each CLAD unit for a cell transfer and the communication controller makes the cell holding portion transfer the cells to the cell terminal portion through the Utopia Level 2 interface to decide whether or not the cells are addressed to itse