An ignition coil for an internal combustion engine is mainly made up of a transformer part and a control circuit part and a connecting part, and the transformer part is made up of a iron core which forms an open magnetic path, magnets, a secondary spool, a secondary coil, a primary spool and a primary coil. By respectively setting the cross-sectional area SC of the iron core between 39 to 54 mm2, the ratio of the cross-sectional area SM of the magnets with the cross-sectional area SC of the iron core in the 0.7 to 1.4 range, the ratio of the axial direction length LC of the iron core with the winding width L of the primary and secondary coils in the 0.9 to 1.2 range, and the winding width L in the 50 to 90 mm range, the primary energy produced in the primary coil can be increased without increasing the external diameter A of the case.