Abstract: To provide an ignition coil for an internal combustion engine which improves high-voltage sealing performance and moreover reduces housing outer diameter, an ignition coil for an internal combustion engine is made up of a transformer portion of cylindrical configuration housed within a housing chamber of a case, a control-circuit portion which is positioned on one end portion of this transformer portion and causes primary current of the transformer portion to be intermittent, and a connecting portion which is positioned on another end portion of the transformer portion and supplies secondary voltage of the transformer portion to a spark plug. The interior of the housing chamber is filled with insulating oil which has a flash point of 180.degree. C. or more, a total acid number of oxidation stability of 0.6 mgKOH/g or less, a pour point of -20.degree. C. or less with no cloudiness occurring before that temperature is reached, and a kinematic viscosity of not less than 20 cSt and not more than 180 cSt at 40.