An ATM communications device connected to an ATM network via an optical fiber improving the efficiency of the traffic in the ATM by separating the real-time data communication system and non real-time data communication system. A CPU bus (or a PCI bus) and a real-time signal bus are connected to an MPEG decoder and an MPEG encoder as real-time data processing devices. The CPU bus is connected to a CPU 18 and a memory 19 as non real-time data processing devices. The ATM communications device includes an optical link, a physical layer device, and an SAR (segmentation and reassembly) devices, carries out processing for communication of real-time data such as a video signal between the ATM network and the real-time signal bus, and carries out processing for communication of non real-time data such as computer information between the ATM network and the CPU bus.