A wiring board according to the present invention has a substrate, a plurality of lines provided on the substrate, an interference-preventive conductor layer provided between the lines to have open ends and prevent signal interference between the lines, and a via electrically connected to the interference-preventive conductor layer. The via is provided at a point at which a distance from each of the open ends of the interference-preventive conductor layer is less than one quarter of a wavelength corresponding to a maximum frequency component of harmonic components contained in the signal.