An ignition coil for an internal combustion engine can increase a contact area between an end of a GND line terminal and a bolt thereby to reduce the contact resistance therebetween. In the ignition coil, a GND potential is ensured by the GND line terminal that is electrically connected at one end thereof to an igniter and at the other end thereof to the internal combustion engine through a bolt inserted into a bolt insertion hole formed in a mounting flange. The GND line terminal is formed at the other end thereof with a bearing surface portion having a hole into which the bolt is inserted. At least part of an inner diameter of the hole in the bearing surface portion is smaller than an inner diameter of the bolt insertion hole.