A signal transmission device of aspects of the invention can include a master circuit connected to the primary sides of first and second transformers and a slave circuit connected to the secondary sides of the first and second transformers. The master circuit sets one of first and second transmitting/receiving circuits for transmitting operation and the other for receiving operation according to a control signal, and detecting a leading edge and a falling edge of the control signal, transmits a pulse signal with the pulse interval changing after a predetermined period of time. The slave circuit detects the change of the pulse interval of the signal received through third and fourth transmitting/receiving circuits and according to the detection result, sets one of the third and fourth transmitting/receiving circuits for receiving operation and the other for transmitting operation.