To limit the arrangement of a sliding core of undercut work only to one place, and reduce manufacturing cost of a metal mold by integrally arranging an installing boss with the peripheral opening edge through curved ribs extendedly arranged in a radial shape from a mold side surface.

A synthetic resin mold 10 is fixed by coming into contact with an installing plate 20 inside of it. In this case, an installing boss 13 is arranged in a flat part 11 of the mold 10, and plural curved ribs 14... are extendedly arranged in a radial shape toward the outside from the side surface periphery of this installing boss 13, and this installing boss is integrally arranged on the opening edge 15a of an opening 15 arranged around the installing boss 13. A shape in an upper surface view of the curved ribs 14 is formed in an almost S shape, and when a tapping screw is screw-fitted to the installing boss 13, the installing boss 13 moves in the direction of an installing seat 21 since the peripheral S-shaped curved ribs 14 are deformed in a condition of being unfolded and extended in an almost straight line shape according to progress of fastening by screw-fitting to the recess shaped installing seat 21 of the installing plate 20 by the tapping screw, and is finally fixed in a condition of being brought into close contact with the installing seat 21.