An optical waveguide is cut by moving a heated knife blade across the optical waveguide with the knife blade maintained at a predetermined angle. At that time, by maintaining the temperature of the knife blade at a temperature not lower than the glass transition point of the optical waveguide, a portion, in contact with a bevel plane of the knife blade, of the optical waveguide softens to be planarized along the bevel plane. In this way, an optical path conversion mirror constituted of a planarized cut plane is formed in a region through which the bevel plane has passed.