An object of the present invention is to provide an ignition coil for use in an internal combustion engine, offering improved water drainage performance, while achieving high waterproof performance with controlled entry of water in a plug hole. A high-voltage generating section 3 that generates a high voltage is housed inside a coil case 4. A plug hole seal 5 closes an opening in a plug hole 2 in which an ignition plug 21 is mounted. The ignition coil further includes a space portion 6 placed on an outer side portion of the high-voltage generating section 3 and an air path 9 for venting air between the space portion 6 and the plug hole 2. The space portion 9 has an open lower surface (an entirely open surface facilitates demolding).