To provide a signal transmission device that completely breaks an input current by determining abnormality if impedance of a load exceeds a predetermined value and returns to a normal operation when the impedance of the load returns to a normal value.

A signal transmission device has an input section and an output section insulatively connected to each other, the input section is connected, via a two-line transmission path, to a host apparatus that includes a current source for outputting a current to the input section and current detection means for detecting an output current of the current source, and the output section is connected to a field device having a predetermined load via a two-line transmission path. Voltage detection means provided in the output section turns off a switch provided in the input section when detecting abnormality in a terminal voltage of the load of the field device, and controls voltage addition means provided in the output section so as to add a predetermined voltage to an input voltage at a voltage/current conversion section provided in the output section.