<p>To reduce the number of optical devices mounted on a transmission path along which an optical main signal runs to thereby reduce the size and the costs and improve a light transmission quality. An optical amplifying unit (11) allows an excitation light to enter an amplifying medium doped with an optical amplifying active material for optically amplifying and an amplified optical signal is output. A reflection type light-variable attenuator (20) uses a reflection mirror (22) to reflect an input beam to generate a reflection beam, provides an MO crystal in a position where an input beam and a reflection beam pass to impart a Faraday rotation angle by an applied magnetic field to thereby change the attenuation amount of an amplified optical signal and convert light partially passed through the reflection mirror (22) into an electric signal as an input monitor light.</p>
