A signal transmission circuit includes a first photocoupler to which a transmission signal is input, an edge detection circuit which is disposed in a primary side of the first photocoupler, the edge detection circuit being configured to detect a rising edge and a falling edge of the transmission signal, and an edge demodulation circuit which is disposed in a secondary side of the first photocoupler, the demodulation circuit being configured to demodulate the transmission signal by using only one of the rising edge and the falling edge of an edge detection signal output from the edge detection circuit via the first photocoupler.