Ignition coil for internal combustion engine

An ignition coil to be directly connected with a plug terminal of a spark plug for an internal combustion engine, comprising:

• an inner coil unit having an inner spool, an inner coil wound on the inner spool and a center core made of magnetic material and housed centrally inside the inner spool;
• an outer coil unit having an outer spool positioned radially outside the inner coil unit and an outer coil wound on the outer spool, the inner and outer coil units being arranged concentrically;
• a secondary terminal to which high voltage induced in one of the inner and outer coils is applied when the other of the inner and outer coils is energized;
• a coil case accommodating a substantial part of the inner and outer coil units;
• a high voltage tower case having a pipe shaped tower case to be coupled with the spark plug and a metal fitting arranged centrally inside the pipe shaped tower case for connecting in circuit the secondary terminal with the plug terminal, the pipe shaped tower case on a side axially opposite to the spark plug being connected with an axial end of the coil case, and the metal fitting being provided with a main body that blocks an opening of the pipe shaped tower case to the coil case so that the coil case and the pipe shaped tower case form an inner space which accommodates axial leading ends of the inner and outer spools on a side of the spark plug;
• and
• Resin insulating material with which the inner space is filled,
• Wherein the outer spool is made of resin material whose bonding strength to the resin insulating material is weak and the axial leading end of the outer spool is positioned axially away from a reference position by a distance equal to or shorter than 60% of a reference length or by a distance equal to or longer than 90% of the reference length, where the reference position is an axial end of the center core on a side of the spark plug and the reference length is an axial length between the reference position and an axial end of the main body of the metal fitting on a side opposite to the spark plug.