An ignition coil for an internal combustion engine includes an annular elastic seal which is tightly attached to a high-voltage tower and a plug installed in the high-voltage tower to be conductible with a spark plug. The elastic seal hermetically seals a gap between the high-voltage tower and the plug and also functions as a buffer to absorb stress, as exerted from the plug on a case of the ignition coil. This enables the case to be reduced in size as a whole without having to partially increase the wall thickness of the case to ensure a required degree of stiffness of the case and also results in improved degree of hermetic sealing between the plug and the high-voltage tower.