To provide an ignition coil which is easy to draw an input/output line to a transformer with improved energy efficiency.

A contact part between an arm part 22 of an auxiliary core 12 and a side core 13 is fitted to all slit edge parts or put with a uniform space from each side of all the slit part. As a result, an inner drawing line from a transformer part 10 can be easily taken through a gap between the arm parts 22, so that connecting face of the arm part 22 joined magnetically to the side core 13 is located near to an edge part of the slit part 14. At the same time, strong magnetic flux at a peripheral edge part is dispersed at a contact face between the side core 13 and the arm part 22 to prevent magnetic saturation at this part.