The invention provides an undercut processing mechanism which is fitted to a molding die for forming a molded product having an undercut portion and which includes a molding core for forming the undercut portion, which mechanism includes: an inclined pin movable in a first direction inclined relative to a die opening direction of the molding die at the time of removal of the molded article from the die, to allow the removal of the molding core from the undercut portion; and a guide section configured to guide the movement of the inclined pin, wherein the inclined pin has a slide portion slidable in contact with the guide section. In which case, the guide section is so shaped as to contact the slide portion over an entire stroke of the inclined pin so that the first direction of movement of the inclined pin can be regulated, and is so shaped as to have a bending rigidity higher in a particular direction of the slide portion.