A substrate with an optical waveguide includes a substrate having a through hole and two sides, a first optical waveguide defined in the though hole of the substrate so as to linearly extend between both sides of the substrate, a second optical waveguide defined on at least one of the sides of the substrate, and direction changing means for changing a transmitting direction of a light signal from either one of the first and second optical waveguides to the other. Each optical waveguide includes a core layer for transmitting the light signals and a clad layer formed around the core layer.