An ignition coil includes a coil part having primary and secondary coils, a center core, and a peripheral core, a connector case part, a seal rubber, and a ventilation passage. The coil part is inserted in a plug hole of an engine, and the case part is outside the plug hole. An axial end of the peripheral core is inserted in an insertion hole of the case part. The peripheral core and an inner wall surface of the insertion hole define a ventilation clearance therebetween. A gap between the case part and the plug hole is sealed with the rubber. The rubber is closely-attached to the plug hole and the case part along its whole circumference. The passage, through which the plug hole is ventilated, includes the clearance and a ventilation hole formed at a certain position of the case part away from the rubber to communicate with the clearance.