A method and apparatus is provided for increasing the packing density and strength of sand for gas-hardening casting molds. The method is characterized by the steps of filling spaces defined by a pattern plate 1 and a mold frame with gas-hardening casting sand 4, depressurizing a closed container 3 after the filled assembly has been conveyed thereinto, introducing air into the container 3, applying a pressure to the upper surface of the casting sand to increase its packing density, and hardening the casting sand by a hardening gas.