An ignition apparatus for an internal combustion engine can prevent dielectric breakdown resulting from voids, thus making it possible to reduce its size. The apparatus includes a primary bobbin, a primary coil formed of a primary coil conductor wound around the primary bobbin, a secondary bobbin arranged in concentric relation to the primary bobbin, a secondary coil formed of a secondary coil conductor wound around the secondary bobbin, an insulation casing receiving therein the primary bobbin, the primary coil, the secondary bobbin and the secondary coil, and an insulating resin filled into the insulation case. Grooves for guiding the insulating resin between the primary bobbin and the primary coil are formed on an outer peripheral surface of the primary bobbin around which the primary coil conductor having a polygonal cross section is wound.