[Object] To allow multiple power line communication (PLC) devices to communicate with one another via power lines without being affected by the power supply state of an uninterruptible power supply unit coupled among the PLC devices.[Solving Means] Multiple modem means 32 for transmitting or receiving a PLC signal are installed, each corresponding to a power receiving terminal 2 and one or more power supply terminals 3 in an uninterruptible power supply unit 1 in a one-to-one manner. Multiple low-pass filter circuit 7 and 18 for eliminating a PLC signal are installed to a power line between each of the multiple modem means 32 and a power supply circuit 10. Control means 40 and 41 transmit a PLC signal based on a PLC signal received by one of the multiple modem means 32 from at least one of the other modem means 32.