A signal transmission device includes a base plate and a photoelectric converter-sealed member. The base plate is structured to have an optical waveguide formed internally, an opening formed to expose the optical waveguide, and a mounting surface provided to allow an electronic part to be mounted thereon. The photoelectric converter-sealed member is placed in the opening of the base plate to include a photoelectric conversion module for making conversion between an electric signal and a light signal and provided by sealing at least the photoelectric conversion module with a predetermined material to expose at least part of a wiring from the photoelectric conversion module on a substantially identical plane with an opening end face of the opening of the base plate. The opening is either a recess or a through hole. This arrangement desirably ensures the higher-speed transmission of electric signals between the photoelectric conversion module and the LSI package.