To almost collapse and discharge core sand from the inner part of a cast product and to shorten the removing time of the core sand by arranging a gas exhaust hole and a compressed air introducing hole communicated with the surface of a mold at a core print and/or a portion corresponding to an intermediate core print in the mold using the sand core.

A nozzle 9 is descended with a cylinder 8, and after press-fixed to the compressed air introducing hole 6, the compressed air is blown and supplied into the sand core 5 through the nozzle 9, and the compressed air introducing hole 6 by opening a switching valve 11. In this way, the compressed air is exhausted to the upper part of the upper mold 3 from the gas exhaust hole 7 through the sand core 5. During this operation, resin for bonding core sand in the sand core 5 is burnt by molten metal heat and the introduced compressed air. Combustion gas is exhausted from the gas exhaust hole 7 while carried on the stream of the compressed air. At the time of burning the binder, the bonding force of the sand core 5 is lost, and since the core 5 becomes easy to be collapsed, at the time of taking out the cast product by shaking out the upper and the lower molds 3, 4, the sand core 5 is simultaneously collapsed.