PURPOSE: To transfer a plurality of kinds of signals by carrier waves of one frequency and to prevent a trouble due to a noise when an insulation degradation signal or the like in a low-voltage electric passage is transmitted toward a message device by using the low-voltage electric passage.

CONSTITUTION: A detection zero-phase current transformer 6 is arranged with reference to a low-voltage electric passage 3 for a nonutility electric receiving installation connected to a high-voltage electric passage 1 via a transformer 2, and information on a zero-phase current or the like is input to a detector 10. In the detector 10, a signal which is detected by a low-voltage-electric- passage-insulation detection circuit 11 is judged by a CPU 13, it is output to an injection transformer 7 as a digital signal or the like via a gate circuit 14, and it is transferred to a message device 20 through the low-voltage electric passage 3. The message device 20 judges the digital signal or the like which is transferred from the detector by removing an inessential signal by using a processing circuit, it displays the signal on a display 27, and it communicates with a monitoring center by using a telephone line.