A high-strength ignition coil that can reduce stress produced in a coil case and prevent a casting material from being cracked when the ignition coil is mounted on an engine block. A coil case has a mounting flange on an outer surface thereof. A coil assembly is housed in the coil case. A casting material is filled into a gap between the coil case and the coil assembly and gaps which the coil assembly has. A plurality of first guide ribs are provided on an inner wall surface of the coil case or an outer surface of the coil assembly, for positioning the outer surface of the coil assembly with respect to the inner wall surface of the coil case. A limited area in which there is no first guide rib is provided on the inner wall surface of the coil case which faces the mounting flange, or the outer surface of the coil assembly which faces the mounting flange via the inner wall surface of the coil case.