A method of manufacturing a synthetic resin core material having a substantially U-shaped cross section which is used in are extruded molding product for holding an opening portion of a vehicle body such as a door or a trunk of an automobile comprises the steps of extruding a synthetic resin to form a core material having a substantially U-shaped cross section, moving the U-shaped core material toward a cutting machine, rotating opposed rotors having blade-shaped portions in the cutting machine, and cutting and forming cut portions or cut slot portions on both lateral sides of the U-shaped core material.