An optical switch includes electro-optic crystal, and electrode unit including a plurality of electrodes arranged on the same plane in electro-optic crystal to extend in parallel to one another. A refractive index of a part of electro-optic crystal is changed by an electric field generated at electrode unit, thereby switching transmission and reflection of light incident on electro-optic crystal. Electro-optic crystal has an entrance surface through which light enters and an exit surface from which light exits. Electrode unit is located between the entrance surface and the exit surface. An angle θx formed between a longitudinal direction of electrodes and at least one surface from among the entrance surface and the exit surface is set near an angle that satisfies the expression θx=90°−Sin−1[ cos(Tan−1(n))] in a wavelength of light to be modulated.