An ignition coil has an outer coil unit, an inner coil unit, a coil case in which the outer and inner coil units are housed and a tower case accommodating lower parts of outer and inner spools. The outer spool is made of resin material whose bonding strength to resin insulating material, with which the tower case is filled, is weak. An 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. With this ignition coil, cracks are suppressed on the resin insulating material opposed to the axial leading end of the outer spool otherwise caused by thermal stress.