PURPOSE:To carry out mold release positively, by forming an undercut forming surface of a slide core such that a side surface of an undercut of a molded item may be slanted against the ejection direction, and perform rounded corner finishing or chamfering an outer edge of a slide core sliding surface that faces the ejection direction. CONSTITUTION:The undercut forming surface 10a of the slide core 10 is slanted such that the side surface 4a of the undercut 4 of the molded item (M) may be slanted against the ejection direction. The outer edge 16a of the slide core sliding surface 16 of the movable mold 8 that faces the ejection direction is rounded or chamfered. The angle of the slant of the side surface 4a is 20-40 deg.. The radius of curvature of the edge 16a is to be 0.5-1.0, and preferably the angle of the chamfer of the side surface 4a is on the order of 45 deg.. After the completion of the molding, and when the mold is opened and the molded item (M) is ejected, a protrusion 4 is slid on the edge 16a of the movable mold 8 as shown so that the molded item (M) is ejected to complete the release positively.