A signal transmission circuit includes, in each of a first circuit connected to a first coil of an insulating transformer and a second circuit connected to a second coil of the insulating transformer, a transmitting circuit, a receiving circuit, a coil-side switching circuit, an input/output-side switching circuit, an abnormality detection circuit, a delay circuit, and a direction control section. In the signal transmission circuit, the direction control section controls the switching circuit to switch a signal direction between input and output, and the switching circuit switches between transmission and reception. The delay circuit delays a received signal and returns the resultant signal to the transmitting side, and the abnormality detection circuit detects abnormality to perform self-diagnosis.