An undercut processing mechanism in a molding device for molding an article to be molded with a stationary mold and a movable mold, the molded article having an undercut portion formed inside it in a circumferential direction thereof at a time of punching out the molded article and brought to a detachable state, comprising: an undercut mold core comprising plural split cores that line one another in an entire circumferential direction to correspond in position to an inner peripheral surface of the undercut portion, the split cores comprising inner split cores and outer split cores formed to line alternately, the inner split cores abutting on one another and formed on an inner side and the outer split cores coming close to one another along an outer side of the inner split cores when the split cores have been reduced in diameter toward an center axis of the undercut portion; slide members radially disposed and opposed to the center axis of the undercut portion in a state in which each of the slide members supports each of the split cores of the undercut mold cores on a leading end thereof; a holder provided integrally with the stationary mold or movable mold for housing the slide members slidably between a molding position at which the split cores supported on the leading ends of the slide members, respectively, line one another in the entire circumferential direction and a demolding position at which the split cores are reduced in diameter one another toward the center axis of the undercut portion; guide means disposed in the holder for guiding, at the time of punching out the molded article, the slide members to move simultaneously in a punching-out direction parallel to the center axis of the undercut portion and in a direction escaping from the undercut portion toward the center axis of the undercut portion, respectively, along an inclination direction from the molding position to the demolding position; and the mechanism being characterized by ejector pins driven in the punching-out direction to project out, wherein the holder is integrally provided with the movable mold in a state in which leading ends of the ejector pins are movably guided, the slide members have proximal ends thereof connected slidably in the escaping direction to the leading ends of the ejector pins, and the guide means comprises guide holes which are formed in the holder and along which the slide members are movably guided in the inclination direction, with midway portions of the slide members passing through the guide holes. an undercut mold core comprising plural split cores that line one another in an entire circumferential direction to correspond in position to an inner peripheral surface of the undercut portion, the split cores comprising inner split cores and outer split cores formed to line alternately, the inner split cores abutting on one another and formed on an inner side and the outer split cores coming close to one another along an outer side of the inner split cores when the split cores have been reduced in diameter toward an center axis of the undercut portion; slide members radially disposed and opposed to the center axis of the undercut portion in a state in which each of the slide members supports each of the split cores of the undercut mold cores on a leading end thereof; a holder provided integrally with the stationary mold or movable mold for housing the slide members slidably between a molding position at which the split cores supported on the leading ends of the slide members, respectively, line one another in the entire circumferential direction and a demolding position at which the split cores are reduced in diameter one another toward the center axis of the undercut portion; guide means disposed in the holder for guiding, at the time of punching out the molded article, the slide members to move simultaneously in a punching-out direction parallel to the center axis of the undercut portion and in a direction escaping from the undercut portion toward the center axis of the undercut portion, respectively, along an inclination direction from the molding position to the demolding position; and the mechanism being characterized by ejector pins driven in the punching-out direction to project out, wherein the holder is integrally provided with the movable mold in a state in which leading ends of the ejector pins are movably guided, the slide members have proximal ends thereof connected slidably in the escaping direction to the leading ends of the ejector pins, and the guide means comprises guide holes which are formed in the holder and along which the slide members are movably guided in the inclination direction, with midway portions of the slide members passing through the guide holes.