Alignment of a plurality of core sections, which diagonally cross each other in a light guide, with a plurality of optical fibers connected to each of the core sections is highly accurately performed. An optical multiplexing/branching device (1) is provided with a substrate (2), and a light guide (4), which is formed on the substrate (2) and optically connected to optical fibers (6a, 6b, 6c). Thelight guide (4) is provided with a plurality of linear core sections (12a, 12b, 12c) diagonally crossing each other. The substrate (2) is provided with V-shaped grooves (28a, 28c), which align the optical fibers (6a, 6b, 6c) optically connected with two or more of the core sections (12a, 12b, 12c) and support the optical fibers. When the optical fibers (6a, 6b, 6c) are supported by the V-shaped grooves (28a, 28c), the center deviation between the core sections (12a, 12b, 12c) and the optical fibers (6a, 6b, 6c) connected with the core sections is 5 mu m.