An optical wiring substrate provides a slab optical waveguide having a refractive index different from a refractive index of other surrounding portions and a planar convex lens being provided continuously to a tip portion of an optical path of an optical wave guide including a core and cladding and formed on a substrate. The optical path of light passing through the planar convex lens is converted approximately by 90.degree. with a mirror. The light reflected with the mirror is made as parallel light rays by use of a cylindrical lens.