To provide stability and a high speed property by preventing the generation of any slave stations, to which transmission is impossible, even if a frequency exceeds a specified level during the control of a lamp using a power line.

The modems A and B of transmission master stations generate control signals made of modulation signals modulated individually by a plurality of frequencies (a) and (b) and send these signals, and the modem of each slave station demodulates the control signal sent from each of the transmission master stations based on any one of the frequencies (a) and (b). Thus, even if one modulation signal is attenuated to disable transmission, the other modulation signal is still receivable.