An optical modulator using a thin plate capable of improving an S/N ratio of an output light is provided. The optical modulator including a thin plate 1 having an electrooptic effect and having a thickness of 20 .mu.m or less, an optical waveguide 4 formed on a top or bottom surface of the thin plate, and a modulation electrode formed on the top surface of the thin plate to modulate light passing through the optical waveguide, wherein a stray light removing member 10 is disposed within the thin plate or in a vicinity of the thin plate.