A signal transmission system which perform two-way signal transmission between a first transmitter-receiver and a second transmitter-receiver via a pair of signal lines and supplies a DC current from the first transmitter-receiver to the second transmitter-receiver. In each of the first transmitter-receiver and the second transmitter-receiver, there are provided a current varying circuit for varying a signal line current by a transmission pulse signal so that the signal line current may assume a value in a predetermined current region lower than a minimum level of a load current to the other transmitter-receiver during sending out a signal pulse, a current detector for detecting a current or voltage variation in the signal line current, and a gate circuit for blocking the detected output from the current detector while the transmission pulse is sending out to the current varying circuit.