An ignition coil has a coil case made of an insulating resin, including a head portion, a cylinder portion accommodating a primary coil, and a secondary coil and a high-voltage tower portion for fitting with a spark plug. At least one of those three portions is separately molded and then the portions are assembled to form the coil case. The longitudinal length of each of the three portions is less than a forming flow length of the insulating resin, thereby preventing inferior molding. Further, the structure of each molding die is simpler than that of the coil case for an integral molding. Therefore, the coil case of the ignition coil can be molded readily without molding defects, even when the coil case is elongated, formed thinner, and/or formed with an undercut portion.