PURPOSE: To eliminate the need for a power supply controlling line, by turning on a main power supply of a master transmitter from a slave transmitter side via a data transmission line.

CONSTITUTION: A master transmitter and a slave transmitter 5 transmit data with transmission lines 9 and 10, a switch 8 is opened at nonoperation of a system and the transmission lines 9 and 10 are connected to a current source 13 and a current detector 14 via a relay 15. When the switch 8 is closed, a closed loop is constituted of the transmission lines 9 and 10, a pulse transformer 7, the current source 13 and the current detector 14. A conduction detecting signal 17 is stored 16, an output 18 turns on a main power supply 3 and given to the relay 15 to connect the lines 9 and 10 to a transmission section 2, allowing to perform data transmission.