PURPOSE: To provide the network communication system which can share transmission from one transmission-side communication terminal device to plural specified communication terminal devices and can also improve economy.

CONSTITUTION: A termination-side communication terminal device once receiving a call setting (SET UP) command with monitor mode specification judges whether its station is a main station 3 or slave station, and sends respective in-call-setting process (Call Proc), in-calling (Alert), and response (Conn) messages in order to a network 7 when its station is the master station 3, and a response confirmation (Conn ACK) message is sent from the network 7 back to the main station 3. Then when the origination side and slave station are in a communication ready state, i.e., when communication data arrive, line monitoring specification is completed through the single main station 3. It is judged that communication terminal devices which are not the main station 3 are slave stations 4...6, and the slave stations 4...6 are specified concurrently and placed in a state wherein received data from the origination side can be shared; and those respective slave station communication devices 4...6 monitor the sent communication data and operate according to the monitored data.