PROBLEM TO BE SOLVED: To promote the disintegration of a hollow sand core in the inner part of a cast product and to disintegrate a sand core in a short time by inserting metallic pieces into the hollow part of the sand core, vibrating the metallic pieces in the hollow part of the sand core and disintegrating the sand core. SOLUTION: The metallic pieces 6 inserted into the hollow part 3 of the sand core 2 is vibrated by resonating the vibration of the cast product 1 generated by hitting a feeder head part 4 of the cast product 1 with a hitting device 5. The metallic pieces 6 inserted into the hollow part 3 of the sand core 2 is resonated to the vibration of the cast product 1 at the time of hitting the feeder head part 4 of the cast product 1 and mutually collided and scattered in the hollow part 3 and collided to the sand core 2, and the sand core 2 is perfectly disintegrated. Since the feeder head 4 is hit, additional hitting part is unnecessary. Further, since the cast product is not directly hit, but the metallic pieces 6 are vibrated by hitting through the feeder head 4, the development of the crack in the cast product 1 is prevented.