An ASE beam, so generated in a first rare-earth element-added optical fiber (8) by an exciting light from a exciting light source (2) as to obtain with a simple structure a light of a desired spectrum, is introduced into a second rare-earth element-added optical fiber (9) and is output with a desired light spectrum. Since the second optical fiber (9) absorbs part of the incident ASE beam to emit a light with wavelength different from that of the absorbed light for synthesizing with a transmitted light to thereby produce a desired spectrum, a large-power output light is obtained with a simple structure.
