A transmission processing apparatus selects, based on flow control information, user packets to be multiplexed, stores the user packets in the payload of a multiplexed packet based on the position order of multiplex flags corresponding to destination users, stores a multiplex flag group in which only the multiplex flags of the user packets are set in the header of the multiplexed packet, and transmits the multiplexed packet. A reception processing apparatus extracts the multiplex flag group from the header of the received multiplexed packet, transmits flow control information according to the storage state of a reception buffer to the transmission processing apparatus, demultiplexes the user packets from the payload of the multiplexed packet, extracts destination user information corresponding to the set multiplex flags from user management information, and outputs the user packets to the destination users corresponding to the user management information.