An apparatus capable of reducing a waveform distortion of outgoing light when light of an optical wavelength in a certain specific narrow range is incident upon optical fiber comprises optical source 10 for supplying the incident light to optical fiber line 110, waveform monitor 42 for measuring a waveform distortion of the transmitted light and adjusting unit 44 for adjusting an output of the incident light so that the measured waveform distortion falls within a predetermined range. By adjusting the output of the incident light, a S/N ration is lowered. Since the noise exists within a relatively wide range of wavelength, the rang of wavelength of the incident light is widened. Therefore, it is possible to reduce the waveform distortion of the outgoing light.