PURPOSE: To switch effectively signals by reducing the counter electromotive force generated at an inductive load by the inversion of polarity of the DC component of a DC/AC current and at the same time preventing the propagation of the counter electromotive force to a current switching point by a current switching contact.

CONSTITUTION: A diode bridge circuit BR1 is used to protect a circuit from the counter electromotive force generated at an inductive load within a signal input/ output device in case the DC and AC signals of a telephone circuit LN are connected. Then the circuit BR1 is put between a relay contact K1 and a coil L1, and the variations of positive/negative polarities of a circuit control signal are transmitted to the coil L1. As a result, the generated counter electromotive force is considerably reduced. In such a way, the generation of noise is prevented. Then the circuit control signal is intermitted with intermission of the contact K1. Therefore, the current is absorbed by the circuit BR1 although the counter electromotive force is generated at the coil L1, the generation of noises is suppressed.