To provide a communication network control system suitable for mutually connecting an existent digital communication network or analog communication network with an ATM communication network, communication network for moving object or Internet.

In a multiplexing/demultiplexing device 40 for multiplexing/ demultiplexing communication information/control signals from the terminal of a moving object or to the terminal of the moving object, a header terminating part 401 receives the communication information from the terminal of the moving object made into ATM cell or communication information/control signals to the terminal of the moving object, terminates a header and outputs it to a conversion table 402 and an analyzing part 403. The analyzing part 403 analyzes CID, LID and RNTI extracted from the header terminating part 401. A multiplexing/demultiplexing part 405 multiplexes/demultiplexes the ATM cell for each kind on the basis of the analyzed result. A header applying part 406 applies the destination of a processor to next perform processing as a header corresponding to the kind of output from the multiplexing/demultiplexing part 405 and outputs the result.