An optical modulator using a thin plate including a portion having 20 .mu.m or less thickness, capable of selecting a specific polarized wave from a light wave which propagates in an optical waveguide in the modulator without reduced productivity. The modulator includes an X-cut or Y-cut thin plate of a material having an electrooptic effect; an optical waveguide formed in a top or bottom surface of the plate; and a control electrode on the top surface of the plate to modulate light which propagates in the waveguide. An attenuating means which absorbs light having a specific polarization plane of a light wave which propagates in the waveguide is disposed in the vicinity of the waveguide. The attenuating means includes light absorbing members disposed at a gap L which is 0.5 to 2 times the mode-field diameter of the light wave which propagates in the waveguide with the waveguide interposed therebetween.