A pair of mold cores (51, 52) for molding an undercut portion (P1) in a holder (30) installed inside a movable mold (13) are connected to and supported by a retaining piece (40) which is moved in the demolding direction, and at the time of demolding, the respective mold cores (51, 52) are guided by guide means (33, 34) from the molding position, where they are contacted with each other, to the mold release position, where they are separated from each other, being moved from one end side to the other end side of the retaining piece (40), in the mutually opposite directions so as to pass each other in the back and forth direction.