The present invention relates to an optically active device comprising a plurality of stages of optical amplifying sections cascaded on an input light propagation path, and a structure for effectively preventing an upstream pumping light source from being destroyed by ASE light propagating in a direction opposite to the input light. The optically active device comprises, at least, a front-stage optical amplifying section and a rear-stage optical amplifying section which are adjacent to each other on the input light propagation path. Each of the front-stage optical amplifying section and rear-stage optical amplifying section includes an amplification fiber doped with ytterbium as an optically active material and a pumping light source for supplying the amplification optical fiber with pumping light in the band of 0.98 μm for pumping the optically active material. In particular, the optically active device comprises a deterioration preventing structure for preventing the pumping light source in the front-stage optical amplifying section from deteriorating a performance. An embodiment of the deterioration preventing structure is realized by a wavelength-multiplexing fiber coupler of a wavelength division type arranged between the front-stage optical amplifying section and rear-stage optical amplifying section. The wavelength-multiplexing coupler has a port for selectively eliminating from the input light propagation path at least an ASE component in the band of 0.98 μm from backward ASE light from the rear-stage optical amplifying section.