To constitute a simple and inexpensive LAN by using a power line by allowing commercial power frequencies to pass through the upstream side of a branch point where wiring is branched into plug sockets, and connecting a filter which prevents signal frequencies for data transmission, and operating the display of a connection plug socket for an outdoor LAN at each plug socket.

Commercial power frequencies are allowed to pass through the upstream side of a terminal board 15 including a branch point 14 at which wiring is branched into m-pieces of plug sockets 13 for data transmission, and a filter 16 which prevents signal frequencies for data transmission is connected. The display of a connection plug socket for an outdoor LAN is displayed at each of the m pieces of plug sockets. Also, it is possible to provide a plug socket 18 not for data transmission which is branched from wiring between the branch point 14 and the plug sockets 13 at which the display for data transfer is operated to allow the commercial power frequencies to pass, and connected through a filter 17 which prevents the signal frequencies for data transmission.