PURPOSE: To provide forming mechanism which is suitable to manufacture a half body of an inserting hole part of a resin pattern for casting a curving deformed tube with a full mold process and easily release the pattern from the mold.

CONSTITUTION: An inner mold 24 and an outer mold 14 for forming the inner surface and the outer surface of the inserting hole part 6 of the resin pattern are arranged. The inner mold 24 is divided into plural number in the circular direction and has plural divided bodies 25 and plural spacers 27 arranged between the each divided end parts 26 in these divided bodies 25. The divided bodies 25 and spacers 27 are simultaneously displaced in the diameter direction so as to expand/shrink in the diameters. As stroke at the time of expanding/ shrinking the diameter of the spacer 27 is larger than the stroke of the divided end parts 26, the divided bodies 25 are shrunk in the diameter without interference with the spacers 27.