To provide a forming method of a die casting formed product having an undercut, with which the undercut shaped part in the die casting formed product is directly formed without needing after-working so as to easily take out this formed product from the die after forming.

An ejecting piece forming a prescribed undercut shaped part 2 is arranged at a part of a movable core 7 and this ejecting piece is movable parallel in the ejecting direction of ejecting pins 20, and after forming the formed product 1 having the undercut shaped part 2 by integrally using the movable die 7 and the ejecting piece, the movable die 5 is opened, and the ejecting piece and the ejecting pins 20 are ejected. Consequently the space is developed for shifting the formed product 1 to a lateral direction in the inner part of and around the formed product 1, and in this state, the formed product 1 is shifted to the lateral direction and separated from the ejecting piece.