An ignition coil includes primary and secondary coils, center and outer cores, and an ion current detector for transmitting an ion current detection output. The center core has an axial upper end defining a center core upper end surface. The outer core has an axial upper end defining an outer core upper end surface. The center core upper end surface axially protrudes upwardly relative to the outer core upper end surface. The center core upper end surface is located at a stagger distance axially from the outer core upper end surface. The stagger distance is defined such that a detection period for residual magnetic noise in the ion current detection output falls within a system requirement period of a control unit.