An undercut processing mechanism, being provided in a mold for molding an article to be molded having an undercut portion with a stationary mold and a movable mold, bringing said undercut portion into a state allowing demolding it, comprising: a holder, being installed inside said stationary mold or said movable mold; a pair of mold cores (51, 52), being housed in said holder for molding said undercut portion, surrounding it from both lateral directions crossing the demolding direction; and a retaining piece, being housed in said holder and being movable in the demolding direction, basal end portions of said respective mold cores (51, 52) being connected to said retaining piece, respectively, so as to be slidable in both lateral directions leaving from said undercut portion crossing said demolding direction, said respective mold cores (51, 52) being movable between a molding position where they are contacted with each other so as to surround said undercut portion from both lateral directions in said holder, and a mold release position where they are projected outside said holder and mutually separated from said undercut portion, in said holder, there being provided a pair of guide means (33, 34), extending mutually symmetrically, for guiding said respective mold cores (51, 52) along an inclination direction upon a movement of said retaining piece for moving them simultaneously in the demolding direction and either of both lateral directions, respectively, from the molding position to the mold release position, said respective mold cores (51, 52) being provided with a geometry allowing at least the respective basal end portions to abut on each other in the back and forth direction with said retaining piece being interposed between them, upon being moved from the molding position to the mold release position, the respective basal end portions being moved from one end side of said retaining piece to the other end side thereof in the mutually opposite directions so as to pass each other in the back and forth direction. a holder, being installed inside said stationary mold or said movable mold; a pair of mold cores (51, 52), being housed in said holder for molding said undercut portion, surrounding it from both lateral directions crossing the demolding direction; and a retaining piece, being housed in said holder and being movable in the demolding direction, basal end portions of said respective mold cores (51, 52) being connected to said retaining piece, respectively, so as to be slidable in both lateral directions leaving from said undercut portion crossing said demolding direction, said respective mold cores (51, 52) being movable between a molding position where they are contacted with each other so as to surround said undercut portion from both lateral directions in said holder, and a mold release position where they are projected outside said holder and mutually separated from said undercut portion, in said holder, there being provided a pair of guide means (33, 34), extending mutually symmetrically, for guiding said respective mold cores (51, 52) along an inclination direction upon a movement of said retaining piece for moving them simultaneously in the demolding direction and either of both lateral directions, respectively, from the molding position to the mold release position, said respective mold cores (51, 52) being provided with a geometry allowing at least the respective basal end portions to abut on each other in the back and forth direction with said retaining piece being interposed between them, upon being moved from the molding position to the mold release position, the respective basal end portions being moved from one end side of said retaining piece to the other end side thereof in the mutually opposite directions so as to pass each other in the back and forth direction.