To form a cast markedly enhanced in relief effect by fixing a relief cast forming instrument along with a foot by the cast so as to form a predetermined gap between a cast foot bottom part and the sole of a foot after fixing when the cast is wound.

A bellows bag 2 is arranged in a protective case 20 made of a cured resin in such a state that an air port 21 is closed and a foot 3 is placed thereon to be wound by a cast by usual procedure. Considerable pressure is applied to the bellows bag 2 when the cast is wound to be solidified but the shape of the bellows bag 2 is held and, when an air port 21 is released after the cast 1 is cured, the bellows bag 2 becomes freely stretchable and a lower leg cast excellent in relief effect is completed. The protective case 20 has the function protecting the flexible bellows bag 2 from the cast 1 wound around the periphery of the foot and the bellows bag 2 can be shrunk in the cast 1 in an up-and-down direction (the long axis direction of the tibia) even by minute pressure by the protective action of the protective case 20.