Abstract: An optical transmission device improved in optical coupling loss between light guides and optical elements and excelling in efficiency of light utilization is to be provided. The optical transmission device is provided with light guides having light incidence/emission sections on plural stepped portions, a substrate which fixes the light guides, and light receiving elements and light emitting elements arranged on the substrate to match the light incidence/emission sections of the light guides. The light receiving elements and the light emitting elements are arranged on the substrate by use of optical connectors. Coefficients of linear expansion and the rates of dimensional variation due to water absorption (or water absorption rates) of the light guides and the substrate here are substantially equalized.