To obtain a destructible core (a core mold) for manufacturing a hollow part which cannot be simply removed from a molding die for hollow parts of various shapes, especially an undercut.

A doughnut-shaped core is manufactured on a rim 1 with a center part for connecting two lateral annular fixing areas. This rim 1 has a selectively openable air feed port connected to a pressure source, an atmosphere and a vacuum source. In addition, the rim 1 has a conduit 3 equipped with a selectively openable valve 30. Further, a freely stretchable membrane is installed on the outside of the radial direction of the rim 1, and is airtightly fitted to the fixing area of the entire outer periphery of the rim 1 and thereby, a closed space is formed between the rim 1 and the membrane. Next, a fluid fixing material 6 is filled into this closed space so that the membrane is tightly fitted to a molding die 5. After this filing process, the membrane is pressed against the mass of the filler under the effects of a vacuum maintained in the closed space to obtain a substantially rigid body core supported by the rim 1.