An optical modulator includes a first graphene and a second graphene on an upper surface of a semiconductor layer, a first electrode on the first graphene, and a second electrode on the second graphene. Respective side surfaces of the first graphene and the second graphene are separated from each other. A first ridge portion of the semiconductor layer and a second ridge portion on the second graphene constitute an optical waveguide, and the first and second graphenes are on a center portion of the optical waveguide in a vertical direction to the semiconductor.