An object of the present invention is to provide an optical device, in which a stress applying on ridge sections caused by the difference of thermal expansion between a metal film of which a ground electrode is formed and a substrate is reduced. In order to achieve this, in the optical device of the present invention, the ratio of the volume of a ground electrode portion formed at the position of other groove to the area of opening part of the other groove is smaller than the ratio of the volume to the area of a ground electrode portion formed at the position except the other groove.