PURPOSE: To obtain a communication system provided with a network terminal device (NT) capable of easily composing the communication system regardless of communication forms, regarding a communication system composed of plural network terminal devices to be connected with an ISDN and plural terminals to be connected with the NT.

CONSTITUTION: On a NT of a transmission side, a decision part 190 discriminating the other party address of a communication inputted in the NT itself, discriminating the other party address by receiving a signal to which the other party address of the communication from a terminal is added and deciding whether a multiplexing is required or not, and a multiplexing/speed conversion part 220 multiplexing an input signal according to whether the multiplexing is required or not and outputting the signal by converting it to a prescribed speed are provided. On the NT of an incoming side, a decision part 250 receiving a signal to which an other party address of a communication is added, discriminating the opposite address and deciding whether a multiplexing and separating is required or not, and a multiplexing and separating/speed conversion part 280 separating the multiplexing of the input signal according to whether the multiplexing and separating is required or not or outputting the signal by converting it to a fixed speed are provided.