To reduce the compensation error of the gain-wavelength dependence of an optical amplifier and the polarization dependence of the transmittance by outputting optical signals of mutually different wavelengths to cascaded etalon filters having mutually different free space spectrum ranges.

Optical fibers 11, 12 are optically coupled through a beam collimated by lenses 21, 22 between which N cascaded etalon fibers 30 having mutually different free space spectrum ranges are inserted. The max. and min. values of the loss and free space spectrum range of the N-th etalon filter are set by developing the gain-wavelength characteristic of an optical amplifier to which an optical equalizer is applied into a Fourier series. The etalon fibers 30 may use one having dielectric multilayer films or metal films having specified refractive indexes on both sides of a glass plate.