A communication unit which transmits packets in the form of ATM cells, includes a packet buffer part receiving Virtual Channel (VC)-multiplexed ATM cells from an input line, judging a packet type, converting a header and assembling a packet, a protocol processing part receiving and processing a protocol packet output from the packet buffer part when the packet type of the packet assembled in the packet buffer part is the protocol packet which is used for communication control, and a packet output scheduling part, including a scheduler which carries out an arbitration when an output request generated upon completion of a reception of a user packet from the packet buffer part and an output request generated upon ending of a process from the protocol processing part are received, and selects and outputs to an output line the user packet or the protocol packet based on the arbitration.