To secure a telephone function by changing a connecting destination of a LAN telephone set/LAN telephone adaptor when power interruption or a network fault occurs in a LAN telephone system.

Each of LAN telephone sets 4-1, 4-2 (LAN telephone adaptor 5) connected to a server through a LAN cable and hub 3 in the LAN telephone system is provided with a link pulse detection circuit 17 for detecting a network fault, an AC power supply detection circuit 16 for detecting the existence of power supply and switches 13, 19 for changing a connecting destination of the LAN telephone set (the LAN telephone adaptor) connected from the hub 3 to a private branch exchange (main wire 10) with a power supply coping with of power interruption at the time of detecting at least either one of a network fault and power interruption.