To provide a wavelength-selection optical switch able to select a light with a certain wavelength and an optical add/drop multiplexer using the same. An optical switch comprising: a silicon substrate; a glass substrate arranged to face a surface of the silicon substrate and having a transparent electrode layer in its opposing surface; a light diffraction reflection layer aligned on a surface of the silicon substrate; a liquid crystal layer aligned between the light diffraction reflection layer and the transparent electrode layer; a silicon substrate electrical terminal extracted from a rear surface of the silicon substrate; and a means for applying voltage between the transparent electrode layer and the silicon substrate electrical terminal; wherein selection of wavelength of light diffracted and reflected by the light diffraction reflection layer is possible by controlling the refraction index of the liquid crystal layer through the voltage. A beam diameter of a wavelength multiplexed light signal is widened by a collimator lens, the wavelength multiplexed light signal with widened beam diameter is directed to the optical switch, a light signal which has wavelength selected through the applied voltage on the optical switch is only diffracted and reflected by the optical switch.