An ignition coil for supplying a high voltage to a spark plug is mainly composed of a cylindrical casing, an outer coil wound around an outer spool and an inner coil wound around an inner spool. The outer coil wound around the outer spool and the inner coil wound around the inner spool are coaxially disposed in the cylindrical casing. Spaces in the cylindrical casing including a first space between the inner coil and the outer spool and a second space between the outer coil and the cylindrical casing are filled with insulating resin. After the resin is supplied into the cylindrical casing, the coils are inserted into the casing containing the resin therein. Accordingly, the resin can be quickly supplied from a wide opening, and the resin can enter into spaces between coils even these spaces are small.