Abstract: An optical semiconductor device includes a substrate and a plurality of optical parts mounted thereon. Guide grooves are formed in the substrate at the mounting positions of the optical parts. Each guide groove has first side surfaces, which are substantially orthogonal to the direction of the optical axis, second side surfaces, which are substantially parallel to the direction of the optical axis and substantially perpendicular to the surface of the substrate, and a bottom surface substantially parallel to the surface of the substrate. Each guide groove has a size and a depth that allow at least a bottom portion of the corresponding optical part to be received therein. The corresponding optical part is accommodated in each guide groove in abutting contact with the first side surfaces of the guide groove, whereby the optical parts are aligned with one another along the direction of the optical axis.