A signal transmission circuit has first to fourth inductors, and first and second varistors. The first and second inductors are magnetically coupled to each other. The first varistor is connected to ground and located posterior to the first inductor and is electrically connected to the first inductor. The third inductor is located between the first inductor and the first varistor and is electrically connected to the first inductor. The second varistor is connected to ground and located posterior to the second inductor and is electrically connected to the second inductor. The fourth inductor is located between the second inductor and the second varistor and is electrically connected to the second inductor.