PURPOSE: To attain efficient asynchronous multiplex to cell data handled at a broad band ISDN of the asynchronous transfer system.

CONSTITUTION: A temporary storage cell data quantity of FIFO memories 111-113 connecting to input ports 101-103 is detected by storage cell data quantity detection means 121-123. A maximum storage FIFO memory detection means 130 specifies an FIFO memory having largest storage cell data quantity and informs it to an FIFO memory data read means 140. The FIFO memory data read means 140 reads cell data from the FIFO memory having largest storage cell data quantity with priority, applies asynchronous multiplexing to the data and gives the result to an output port 104. Thus, the asynchronous multiplexing of the cell data is efficiently realized with less FIFO memory capacity.