A light source apparatus for use with optical communication using external-injection light sources, and an optical communication system using the light source apparatus. The light source apparatus for optical communication includes external-injection light sources, and a multiplexer/demultiplexer. The multiplexer/demultiplexer provides light components spectrum-sliced broadband light from a broadband light source as respective external injection light inputs. The multiplexer/demultiplexer includes a first waveguide to transmit the broadband light and multiplexed light with a plurality of wavelengths, a plurality of second waveguides each to transmit light with an associated one of the wavelengths, and a plurality of optical filters each to pass light with an associated one of the wavelengths and to reflect the remaining wavelengths. The pass wavelength of each optical filter is set such that insertion loss distribution of the optical filters at the wavelengths is similar to a gain distribution of the injection light sources at the wavelengths.