The present invention is directed to a gain equalizer having a preferable equalization
characteristic and a structure that can be easily fabricated. The gain equalizer
flattens a spectrum of light in a predetermined wavelength range inputted through
an input terminal and outputs the light from an output terminal, and comprises
a coarse-tunable equalizing section and a fine-tunable equalizing section connected
in series. The coarse-tunable equalizing section coarsely flattens the spectrum
of the light in the predetermined wavelength range, and includes a plurality of
filters each having a large loss and a small reflectance as compared with the fine-tunable
equalizing section. The fine-tunable equalizing section flattens the spectrum of
the light in a wavelength range where the coarse-tunable equalizing section can
not flatten at a predetermined value or less among the predetermined wavelength range.