The present invention relates to a manufacturing method for an optical multiplexer provided with: a substrate having a first main surface and a second main surface that are parallel to each other; a mirror disposed on the first main surface; and an optical filter disposed on the second main surface. This method includes: a step for placing the mirror on the first main surface of the substrate, performing angular adjustment between the substrate and the mirror using an autocollimator, and then fixing the mirror to the substrate; and a step for placing the optical filter on the second main surface of the substrate, performing angular adjustment between the substrate and the optical filter using the autocollimator, and then fixing the optical filter to the substrate.