PROBLEM TO BE SOLVED: To provide a network system using PLC communication for performing stable communication even when many terminal units are connected. SOLUTION: The pre-branch port of a first branching adapter 15 is connected to the communication line connection port of a PLC master unit modem 13, wherein the communication line connection port of a PLC repeater 16 is connected to the post-branch port of the first branching adapter 15, and the pre-branch port of a second branching adapter 7 is connected through a power line to the power line connection port of the PLC repeater 16. Further, the communication line connection port of a PLC slave unit modem 9 is connected through a communication line to the post-branch port of the second branching adapter 7, and the terminal unit 8 is connected to the terminal unit connection port of the PLC slave unit modem 9. Also, a master frame for performing data transfer includes an emergency report band for transmitting information relating to an emergency report obtained from the terminal unit 8 by the PLC slave unit modem 9. COPYRIGHT: (C)2010,JPO&INPIT