An optical waveguide including a core portion configured to propagate light and a clad portion having a refractive index lower than that of the core portion. The clad portion has an underclad portion located on one surface of the core portion. In a cross-section of the core portion perpendicular to light propagation, (i) H/L is 0.4 to 3, or (ii) a taper angle of the core portion is 50° or more and 130° or less, with the proviso that a width direction is a direction of a line along which the core portion and the underclad portion are in contact with each other, L [μm] is a maximum width of the cross-section of the core portion, and H [μm] is its maximum thickness, and the taper angle of the core portion is formed by the underclad and a tangent line of a core side portion.