Disclosed is an optical semiconductor device which can be improved in light shift precision and restrained from undergoing a loss in light transmission. In this device, an inner side-surface of a first optical coupling portion of an optical coupling region and an inner side-surface of a second optical coupling portion of the region are increased in line edge roughness. This manner makes light coupling ease from a first to second optical waveguide. By contrast, the following are decreased in line edge roughness: an outer side-surface of the first optical coupling portion of the optical coupling region; an outer side-surface of the second optical coupling portion of the region; two opposed side-surfaces of a portion of the first optical waveguide, the portion being any portion other than the region; and two opposed side-surfaces of a portion of the second optical waveguide, the portion being any portion other than the region.