An undercut processing mechanism that is installed in a molding die assembly configured to form a molded article having an undercut portion so as to allow demolding of the undercut portion, 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 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; and a retaining piece configured to retain the sliding piece such that the sliding piece is slidable.