A low-cost compact optical attenuator structure has an input port and an output port for optical signals arranged side by side; a reflecting member arranged to face both ports to reverse light from the input side on an optical path folded 180.degree.; a birefringence crystal splits incident light into two polarization components, a converging lens converges the incident light, a fixed Faraday rotator having a variable rotation angle arranged in that order in the optical path. The partial reflecting member that transmits a portion of incident light to the rear side thereof, an optical sensor at the rear side of the reflecting member, optical guide means guide only the transmitted light from the input port side to the optical sensor, and optical detection signals of the optical sensor are used as optical input monitoring signals.