A short cell multiplexing is provided for chiefly transmitting data shorter than the payload of a standard ATM cell (basically the data of less than 48 bytes, but the data more than 48 bytes can be allowed). A standard ATM cell assembler 1 converts various forms of input information into short cells, places the short cells in standard ATM cells efficiently considering their information length, and output them to a B/ISDN network 7. The standard ATM cell disassembler 2 disassembles the standard ATM cells, which are assembled by the standard ATM cell assembler 1 and is input through the B-ISDN network 7, into short cells, converts the short cells into those with the original input information forms, and outputs them to channels. The configuration makes it possible for the short cell ATM cell multiplexing to improve channel efficiency with small delay, and matching to the standard ATM system.