PURPOSE: To permit easy control of a final solidification region and to obtain a defectless casting structure by using a header disposed with many blowing nozzles for the inside surface of a hollow core disposed at the center of a master mold to a gas cooling means for said core.

CONSTITUTION: Cooling gas 10 introduced into a pressure header 13 attached with many nozzles 12 on the outside surface from the upper part thereof collides against the inside surface of a heat-conductive rigid cylindrical body 5 in the form of extremely fast jets 14 in the outlets of the many nozzles 12. Not only high heat conductivity is thus obtd. but also the jets 14 of a low temp. are blown to said surface irrespectively of the position in the height direction of the nozzles 12 and therefore efficient cooling is possible. The gas heated to a high temp. as a result of cooling the surface to be cooled ascends by passing among the jets from the nozzles 12 and is discharged from the upper part.