PURPOSE: To enable the molding of a deeper undercut by performing demolding by repeating operation such that molding is performed by using a molding core consisting of an inner core and plural groups of segments and all of the segments engaged with the undercut are subsequently moved in an axial direction and only one group is moved in a radial direction.

CONSTITUTION: The segments of two different groups 7, 8 are mutually moved with respect to an inner core 5 and, at a time of demolding, at first, all of the segments are together moved in an axial direction. Next, only the segment of the first group 8 is inwardly moved in a radial direction to make an undercut free. Next, the segment of the second group 7 is again moved over a predetermined distance in the axial direction and subsequently moved inwardly in the radial direction. When two or more segment groups are present, all of the segments make the undercut free and, until a product can be removed from the inner core 5 and the segments, the movements in the axial and radial directions are repeated.