PURPOSE: To eliminate necessity of providing an AC power equipment in a slave station and reduce the cost of equipment by transmitting alternating signals for electric power from a master station to a transmission line and making a slave station obtain DC power source basing on the alternating signals for power.

CONSTITUTION: A master station 1 is made to output alternating signals Sa..., Sb... in addition to reset pulse P0, count pulses P1∼P5 to transmission lines La, Lb. These are rectified and constant voltage stabilized in a slave station 12 to obtain a DC power source. Thus, necessity of providing an AC power source equipment to obtain a DC power source in the slave station is eliminated and the slave station can be made small and low cost, and the cost of equipment can be reduced as a whole. Further, as the area that transmits data signals and the area that transmits alternating signals for power are divided by count pulse, transmission of data signals can be made surely.