PURPOSE: To prevent the dropping off of a screw used for a mold member by providing a hole having a taper surface where the side, where a screw is inserted, of a screw hole is widen, and specifying a relation among the minimum diameter of a screw hole, the diameter of the round rod of a screw, and the diameter of a thread ridge.

CONSTITUTION: A relation among the minimum diameter N of a screw hole, the diameter N of the round rod of a screw, and the diameter M of a thread ridge is set to K<N<M. When a stepped screw 1 is inserted in a screw hole 5 in a boss shape of a mold material 6, since the stepped screw is induced in the screw hole 5 along the taper-form surface 9 of the screw insertion hole 8 of the mold material 6, for example, even a machine screw is prevented from falling and insertion is effected with the diameter N of an insertion hole 10 as plastic deformation is effected. Thus, after the stepped screw 1 is inserted in the boss-shaped screw hole 5 of the mold material 6, in a thread ridge containing part 11, the maximum diameter M of the screw part 4 of the screw 1 is increased to a value higher than the minimum diameter N of the screw hole 5 and diameter P is higher than the diameter M of the screw part 4. Thereby, after the screw 1 is mounted, the screw is prevented from slip-out from the insertion diameter 10 and slip-out preventing structure having a play in a vertical direction is provided.