An ignition coil for an internal combustion engine is equipped with an assembly of a connector casing and a coil body. The coil body includes a primary winding wound around a primary spool. The connector casing has terminals each of which is equipped with a conductor fastener. The conductor fastener has a slit in which one of ends of the primary winding is fit to make an electric connection between the terminal and the primary winding. The primary spool has conductor guides and a backup support to establish alignment of each of the ends of the primary winding with one of the conductor fasteners and also to facilitate insertion of each of the ends of the primary winding into one of the conductor fasteners when the coil body is fitted into the connector casing, thereby ensuring the stability of electric connection between the primary winding and the terminal.