A substrate for an optical device includes an optical device substrate including a plurality of conductive plates elongated along a length direction, wherein side surfaces of the conductive plates are bonded to each other with insulators interposed therebetween, the insulators being respectively formed on the side surfaces. A groove having a predetermined depth for preventing burrs is formed in a lower surface of the optical device substrate at each point where a cutting line is crossed with one of the insulators when the optical device substrate is cut in a length direction and in a vertical direction, the groove being formed in such a way that said one of the insulators is exposed to an inside of the groove.