PURPOSE: To make it possible to transmit data reliably by feeding a direct current as the power source for a slave station to a slave station equipment, which has no operating power source, from the master station side and using a telephone line to drive the slave station equipment by the direct current.

CONSTITUTION: Master equipment 01 for data transmission is connected to exchange 03, and telephone set 05 is connected to slave station telephone line 04. Here, diode 07 is conductive only when the voltage of line 04 has one polarity, and Zener diode 08 is cut off below the power source voltage for service and operates to maintain a fixed reference voltage only when the voltage is higher than the reference voltage above. Then, data transmission part 09 is started in DC by the operating voltage which is obtained by subtracting the voltage maintained by Zener diode 08 from the voltage higher than the power source for service given to line 4 only at a data transmission time, so that part 09 can perform a reliable data transmission.