A starter system for an internal combustion engine having a casing provided with a mounting circumferential surface adapted to be substantially closely fitted into an associated mounting bore provided in the engine or a transmission housing associated therewith, and a mounting flange extending radially from the casing and provided with a mounting end surface perpendicular to a longitudinal line of the output shaft of the starter system and spaced from the mounting circumferential surface along the longitudinal line. Because of the presence of the distance between the mounting end surface of the mounting flange and the mounting circumferential surface of the starter casing closely fitted into the mounting bore, the reaction force acting on the pinion is prevented from being amplified and transmitted to the mounting surfaces by means of an undesirable lever action.