PURPOSE: To provide a data transmission circuit in which a filter with a large L and a large C is not required and no radiation noise is generated.

CONSTITUTION: The circuit is provided with a constant current circuit comprising a field effect transistor FET1, a resistor R2 and a Zener diode ZD1 which supplies a prescribed current from lines X1, X2 independently of the operation/ inoperation of an input output device 3 such as a display lamp (LED, switch) connecting to a slave 2 to make the current supplied from a master 1 to the line constant and with a constant current circuit receiving a current from the line and changing the current supplied from the master 1 to the line and comprising a transistor 24 to eliminate an LC filter and to avoid generation of radiation noise.