An optical waveguide includes a laminate including a lower cladding, a core on the lower cladding, and an upper cladding positioned on the lower cladding and covering the core, via holes positioned in the laminate in a spaced opposing relation to each other, a cavity positioned over a span from an upper surface of the upper cladding to the lower cladding, the cavity including a sectional surface sectioning the core obliquely relative to the upper surface of the upper cladding, and a reflective surface positioned in the core and defined by part of the sectional surface, wherein the cavity extends from a region between the via holes in the spaced opposing relation toward the outside of the region, and an opening size of the cavity in the region is smaller than an opening size of the cavity outside the region when viewed in an opposing direction of the via holes.