An ignition coil includes a coil body transversely arranged outside of a plug hole, and a plug connection portion protruding in a direction perpendicular to an axial direction of the coil body. A secondary coil of the coil body has a maximum outer diameter portion, and an inclined outer diameter portion arranged on two sides of the maximum outer diameter portion in the axial direction. The secondary coil includes a secondary electric wire that is wound obliquely around an outer periphery of a secondary spool such that reduced winding parts and enlarged winding parts are alternately superimposed on each other. The reduced winding part is wound with a winding diameter decreasing obliquely from one side to the other side in the axial direction, and the enlarged winding part is wound with a winding diameter increasing obliquely from the other side to the one side in the axial direction.