An optical element mounting method includes: illuminating ultraviolet light onto a polymer optical waveguide device; under the ultraviolet light illumination, capturing, by an image pickup device, the polymer optical waveguide device including a light incident/exiting position of a waveguide core; and judging, from a difference between bright and dark in a captured image, that a portion brighter than other portions or a portion darker than other portions is the light incident/exiting position of the waveguide core.