A method of releasing an inner mold from a rubbery elastic molding having a corrugation in its inner longitudinal cavity and adapted suitably for, but not limited to, use in an electrical bushing or electrical insulator, which method comprises steps: forming a partial space connecting to a pressure source at an interface between the molding and the inner mold; supplying a pressure medium such as a gas or a liquid from the pressure source into the interface via said space to form a desired space between the molding and the inner mold over its entire zone and to expand the inner cavity of the molding; and releasing the inner mold from the molding.