A power line communication system includes a master side transceiver of a master communication device, a slave side transceiver of a slave communication device, a power wire, and a ground wire. The master communication device includes a voltage changeover portion changing supply voltage in steps. The master side transceiver is connected to the slave side transceiver through the power wire and the ground wire. The slave side transceiver transmits first information to the master side transceiver by a differential transmission. The voltage changeover portion changes the supply voltage so as the master side transceiver to transmit second information to the slave side transceiver. A master communication device includes a master side transceiver and a voltage changeover portion.