An optical modulator capable of efficiently separating radiation light and propagation light from each other or removing the radiation light in the optical modulator to suppress loss of the optical modulator and deterioration of an extinction ratio, including a thin plate of a material having an electrooptic effect and a thickness of 20 .mu.m or less; an optical waveguide formed in a top surface or a bottom surface of the thin plate; and a modulation electrode formed in the top surface of the thin plate to modulate light which propagates in the optical waveguide, the optical waveguide has an optical junction portion in which a plurality of optical waveguide portions are joined together and, for shielding a portion of radiation light radiated from the optical junction portion, a concave portion or through-hole in the thin plate.