An undercut processing mechanism that is installed in a molding die assembly configured to form a molded article having an undercut portion (P1, P2, P3, P4) so as to allow demolding of the undercut portion (P1, P2, P3, P4), the undercut processing mechanism comprising: a pushing piece movable in a demolding direction of the molded article; a support element fixed to the pushing piece or formed so as to be integrated with the pushing piece and capable of supporting the undercut portion (P1, P2, P3, P4) during movement of the pushing piece; a sliding piece configured to be slidable relative to the pushing piece and to move, in conjunction with movement of the pushing piece, in a direction intersecting the demolding direction of the molded article; a retaining piece configured to retain the sliding piece such that the sliding piece is slidable, and a holder fixed to the molding die assembly or formed so as to be integrated with the molding die assembly, the holder being configured such that, among components forming the undercut processing mechanism, other components excluding the holder are partially or fully housed in the holder in the time of die clamping; wherein the mechanism is configured such that when the pushing piece is moved in the demolding direction of the molded article, the sliding piece moves in conjunction with the pushing piece while the support element supports the undercut portion (P1, P2, P3, P4), so that the undercut portion (P1, P2, P3, P4) is allowed to be demolded. that is installed in a molding die assembly configured to form a molded article having an undercut portion (P1, P2, P3, P4) so as to allow demolding of the undercut portion (P1, P2, P3, P4), the undercut processing mechanism comprising: a pushing piece movable in a demolding direction of the molded article; a support element fixed to the pushing piece or formed so as to be integrated with the pushing piece and capable of supporting the undercut portion (P1, P2, P3, P4) during movement of the pushing piece; a sliding piece configured to be slidable relative to the pushing piece and to move, in conjunction with movement of the pushing piece, in a direction intersecting the demolding direction of the molded article; a retaining piece configured to retain the sliding piece such that the sliding piece is slidable, and a holder fixed to the molding die assembly or formed so as to be integrated with the molding die assembly, the holder being configured such that, among components forming the undercut processing mechanism, other components excluding the holder are partially or fully housed in the holder in the time of die clamping; wherein the mechanism is configured such that when the pushing piece is moved in the demolding direction of the molded article, the sliding piece moves in conjunction with the pushing piece while the support element supports the undercut portion (P1, P2, P3, P4), so that the undercut portion (P1, P2, P3, P4) is allowed to be demolded. a pushing piece movable in a demolding direction of the molded article; a support element fixed to the pushing piece or formed so as to be integrated with the pushing piece and capable of supporting the undercut portion (P1, P2, P3, P4) during movement of the pushing piece; a sliding piece configured to be slidable relative to the pushing piece and to move, in conjunction with movement of the pushing piece, in a direction intersecting the demolding direction of the molded article; a retaining piece configured to retain the sliding piece such that the sliding piece is slidable, and a holder fixed to the molding die assembly or formed so as to be integrated with the molding die assembly, the holder being configured such that, among components forming the undercut processing mechanism, other components excluding the holder are partially or fully housed in the holder in the time of die clamping; wherein the mechanism is configured such that when the pushing piece is moved in the demolding direction of the molded article, the sliding piece moves in conjunction with the pushing piece while the support element supports the undercut portion (P1, P2, P3, P4), so that the undercut portion (P1, P2, P3, P4) is allowed to be demolded.