An optical system comprising a substrate and an optical waveguide which is formed on the substrate and to which optical fibers are optically coupled. The optical waveguide has a plurality of straight core portions which obliquely intersect each other. The substrate has positioning sections for positioning a plurality of optical fibers optically coupled to two or more of the plurality of the core portions, the positioning sections having grooves on which the respective optical fibers are supported. When the plurality of optical fibers are supported on the respective grooves, offsets between centers of the plurality of the core portions and respective centers of the plurality of the optical fibers coupled to the core portions are equal to or less than 5 .mu.m.