A method of reusing core sand includes: crushing a core used for casting into granules; heating the granules at a temperature of 300 C. to 550 C.; causing the heated granules to collide against each other such that water glass used as a binder detaches from the core sand; and blowing air into a mixture of the water glass and the core sand, which are detached from each other, such that the core sand is separated and collected from the mixture due to a difference in specific gravity between the water glass and the core sand.