An ignition coil includes a primary coil, a secondary coil, a spool, and a center core. The primary coil and the secondary coil are arranged substantially coaxially with each other. The secondary coil is on an inner side of the primary coil. The secondary coil is wound around the spool. The center core is inside of the spool. The spool has a tapered inner surface, which is axially defined at least partially in the spool. The tapered inner surface has the diameter that increases as being distant from a high voltage tip end of the secondary coil on the high voltage side of the secondary coil. The center core has a tapered outer surface, which is axially defined at least partially in the center core. The tapered outer surface has the diameter that increases as being distant from the high voltage tip end of the secondary coil.