PURPOSE: To eliminate the need of laying a transmission line separately from a power line by connecting the power line with transmitter-receivers in electrically insulated states and performing communication by using the power line as a transmission line.

CONSTITUTION: Connecting means which connect transmitter-receivers 2 with a power line 5 in such a state that they are electrically insulated from each other and signal communication can be performed between them are provided and data communication is performed by using the power line 5 as a transmission line. Namely, each data terminal equipment 1 is connected with each transmitter-receiver 2 and receives communicating status information and receiving data from each transmitter-receiver 2. When a transmittable status is recognized, each equipment 1 sets transmitting data to each transmitter-receiver 2. The transmitter-receivers 2 are respectively connected with photocouplers 3. The photocouplers 3 are connected with the power line 5 through signal lines 4 and perform signal communication by electrically insulating the transmitterreceivers 2 and power line 5 from each other.