PURPOSE: To thin a mold by fitting the porous vent plate whose upper part is opened to the specified depth position and providing the backup layer forming tank where the feeding pipe of a compressed air and vent port for vacuum exhaust are arranged.

CONSTITUTION: A refractory powder or metal powder is accumulated in a specified amt. on a vent plate 8 as a backup material powder 15 and a compressed air is fed from an air feeding port 9. The backup material powder 15 on the vent plate 8 becomes in a floating state and in this state a plaster mold 16 is exposed at a sprue part 16a and embedded in the backup material powder 15. The air feeding port 9 is then closed, a sheet 13 like a vinylidene chloride is covered on the upper opening part, an exhaust port 10 is opened and the tank inside is evacuated. The backup material powder 15 is closely fitted to form the backup layer for the plaster mold 16, pouring a molten metal 17 from the above of the sheet 13 into the sprue 16a of the mold 16. A hole is made on the sheet 13, the molten metal is poured in a cavity 16b and vacuum casting is executed. The backup of the mold is thus made complete and the thinning of the mold is realized.