In a state in which a front end of a single-core fiber is passed through an end surface portion of a reinforcing pipe, an inner surface of the reinforcing pipe and an outer peripheral surface of the single-core fiber are glued together by means of adhesive. Envelope detector detects the amplitude of an envelope of the output signal from an orthogonal modulator when the test signals represented by four points are generated, and outputs a signal proportional to the square of the amplitude. The AC electrical fitting engages an AC screw-in type receptacle.