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Waste molding sand discharged from the casting process was treated and reformed using hightemperature and high-pressure water. The surface of the waste molding sand was stained with 1.2 wt% of carbon and 0.065 wt% of nitrogen. Carbon and nitrogen that were removed from the surface by hightemperature and high-pressure water reaction at 300℃, 8.6 MPa were 30 wt% and 64 wt%, respectively. Under the same conditions, with H2O2 as an oxidizer, these removal rates were both 64 wt%. In the reaction at 450℃, 40 MPa, these removal rates were similar to those at 300℃, 8.6 MPa. However, in the reaction at 450℃, 40 MPa with an oxidizer, these removal rates were 88 wt% and 100 wt%, respectively. The acidity value had doubled because of the reaction with high-temperature and high-pressure water on the sand surface. However, it did not increase when the oxidizer was used in the reaction.