PURPOSE: To improve the S/N, by transmitting "1", "0" of a transmission data with a specific character pattern at the transmission side and checking whether or not the received character pattern is coincident with the pattern of "1" or "0" at the receiving side.

CONSTITUTION: A bit "1" of the transmission data is frequency-modulated 3 into the 1st frequency fL and a bit "0" is into the 2nd frequency fH at the transmission side TX. One character is formed with plural bits of the transmission data to transmits the data "1" and "0" to a power line 1. On the other hand, only two kinds of patterns of the characters of the data "1" and "0" are made effective at the receiving side RX, the receiving data "1", "0" are recognized in response to the pattern extracted from the power line 1 and in other cases, the patterns are regarded as the communication error or neglected.