To eliminate the need for a core removing process with an air hammer and a binder incineration process, and to remove the core without sand blasting or shoot blasting by putting the casting, in which the core is built, in the water and spraying the pressure water to the core so as to remove the core.

A casting is submerged in the water in a water tank and fixed. A pressure water is jetted from a nozzle and sprayed to a core inside the casting. The pressure of the water is desirably 100-300 kgf/cm2. When a skirting board part of the core is completely cast, the core is pulled out from a die. Further, the skirting board part is cut to expose the core inside the casting, and the pressure water is sprayed. It is desirable that an appropriate amount of abrasive is mixed into the water in the water tank in advance, or the abrasive to be supplied from an abrasive supplying port arranged near the nozzle is put in the flow of the pressure water and mixed/suspended in the pressure water. The abrasive is desirably the same kind of sand as that constituting the core.