A mobile communication system introducing ATM, AAL2 and IMA techniques in order to efficiently transmit high-speed information and multimedia traffic having various properties, and to increase transmission efficiency in a limited transmission link or transmission band. An ATM cell processing device for a mobile communication system comprises an ATM interface for receiving ATM cells dispersedly transmitted or distributed over a plurality of physical links and restoring the received ATM cells into one ATM cell stream; an AAL2 (ATM Adaptation Layer 2) cell processor for switching at least one AAL2 packet multiplexed within each ATM cell in the ATM cell stream according to routing information provided during call setup, multiplexing the switched AAL2 packets according to ATM connections, and creating an internal ATM cell having the same format as the received ATM cells; and an ATM switch for switching the internal ATM cell according to the routing information.