A light-emitter emits light into a door opening of an automatic door system. A light-receiver receives light from the light-emitter which has passed through the door opening and develops a light-reception indicative signal. A door controller causes the light-emitter to stop emitting light and, thereafter, start emitting light. An auxiliary sensor is judged to be operating in order when the light-receiver does not develop light-reception indicative signal in a state where the light-emitter is not emitting light, and, thereafter, develops a light-reception indicative signal when the light-emitter resumes emitting light; otherwise, the auxiliary sensor is judged to be out of order.