A method of forming an undercut which is engageable with an edge, on a side wall of a cover intermediate body using a cover intermediate body and a secondary processing die. In the secondary processing die, a moving inner core has, on the outer surface thereof, a concave shape corresponding to a convex shape of the inner surface of the undercut. The moving inner core is made up of a plurality of core segments which are arranged along the circumferential direction. Each core segment is vertically movable along an inclined surface of a core. When the moving inner core is moved upward, the moving inner core transforms smaller than the inner diameter of the side wall of a cover. The slide core has, on the inner surface thereof, a convex shape corresponding to the concave shape of the undercut.