A method for manufacturing an optical transmission device, includes: arranging a plurality of optical waveguides including waveguide mirrors, a transmission-side optical module and a reception-side optical module on one side of a substrate; photographing, with a photographic device, at least one waveguide mirror, and the transmission-side optical module or the reception-side optical module corresponding to the waveguide mirror, from another side of the substrate via an opening formed in the substrate; detecting optical-axis centers of the transmission-side optical module or optical-axis centers of the reception-side optical module, and central positions of reflective surfaces of the waveguide mirrors corresponding to the detected optical-axis centers, from a result of the photographing; and aligning and fixing a position relationship between the optical waveguides and the transmission-side optical module or the reception-side optical module based on a result of the detecting.