An undercut processing mechanism can facilitate punching-out of undercut portions having various shapes spreading in the circumferential direction on the inside of a molded article. Split cores (61) that form an undercut molding core (60) are supported one by one at the leading end of slide members (50) disposed radially to face the center axis of an undercut portion (P3). Each slide member (50) is housed in a holder (40) so as to be slidable between a molding position where the split cores (61) are mutually disposed in the entire circumferential direction and a demolding position where the split cores (61) are mutually reduced in diameter. The holder (40) is provided with a guide member (46) for guiding each slide member (50) to move simultaneously in the punching-out direction parallel to the center axis of the undercut portion (P3) and in the direction escaping from the undercut portion (P3) toward the center axis of the undercut portion (P3), respectively, along an inclination direction from the molding position toward the demolding position.