Successive received asynchronous transfer mode (ATM) data cells are stored in a cell reception buffer. All address information included in the received ATM cells in an address table are registered, and the same address information in the address table are updated when there is idle time for a process of assembling packet data from the ATM cells or when there is no empty area in the address table. In a packet reception buffer successive received packet data are stored. The packet data stored in the packet reception buffer are disassembled into ATM data on the basis of address information retrieved from the address table.