PURPOSE: To prevent the occurrence of defect caused by blow hole in the bulging part in a cast formed product by communicating the one end in the inner part of an upper core mold with the atmosphere and providing a gas guiding hole communicated with a shell core at the other end thereof.

CONSTITUTION: By mounting the shell core 5 for forming a bulging part at a prescribed position of the upper core mold 2 fixed with a shell pin 6 for restraining the core, the upper core mold 2 is set in the mold 1 to execute the cast formation. Therefore, the shell core 5 is firmly fixed to the prescribed position, and the bulging part 21 and the bottom part 22 are precisely formed in the decided dimension and further, the whole thickness is uniform and the high quality product is obtd. On the other hand, when the shell core 5 is exposed to the high temp. molten metal, the air in the pore in the inner part is expanded or gas is generated due to thermal decomposition of a synthetic resin. Since the gas guiding hole 7, in which the upper end part is communicated with the atmosphere and the lower end part reaches the shell pin 6, is bored at the upper and the lower directions in the inner part of the upper core mold 2, the generated gas, etc., are released from the gas guiding hole 7 through the pore of the shell pin 6.