An optical axis adjustment method for optical interconnection, includes: providing, on a substrate, an optical transmitter including light sources and a mark for acquiring a position of each of the light sources; providing, on the substrate, an optical waveguide including cores each allowing light emitted from the respective light sources to propagate through the core; determining a first position based on the mark as a position of each of the light sources; and forming, at a second position in the optical waveguide corresponding to the first position, first mirrors configured to reflect the light emitted from the respective light sources and make the light propagate through the respective cores.