PURPOSE: To form a very fine through-hole having a desired shape and size easily in the desired position of a casting product, by installing a fine wire in a mold and pouring molten metal in the mold in the stage of casting the casting product and drawing the wire in the stage of solidification.

CONSTITUTION: A fine wire 4 consisting of a material which is not melted and eroded by molten metal, for example, ceramics or the like and having a desired diameter is installed in a master mold 1 from the inside of a hollow core 2 and the wire 4 is kept applied with tension prior to pouring of molten metal into a mold. Thereafter, molten metal 8 is poured into the mold and the stage in which the molten metal solidifies in the mold is monitored by measuring temp. The taking up of the wire 4 is started with a take-up device 7 under application of prescribed tension thereon from the point of the time when the metal 8 solidifies to a half molten state, whereby the wire 4 is removed from the inside of the mold. A very fine hole is thus formed in the desired position in the casting product which is completed of solidification.