In a signal transmission device having a pulse generator, a RS F/F circuit and a detector, the generator generates a set pulse signal and/or a reset pulse signal when a state of a PWM signal is changed. After the generation of the set pulse signal, the generator continuously generates following pulse signals after elapse of a predetermined period of time counted from the generation of the set pulse signal. The generator adjusts, based on a selector signal, the predetermined period of time counted to a time when the following pulse signal is transmitted at a first time. The detector detects the state of the selector signal based on the predetermined period of time counted from a time when the RS F/F circuit receives the set pulse signal or the reset pulse signal to a time when receiving the following pulse signal at a first time.