The objects of the present invention are to solve the problems wherein the properties of the foundry sand vary due to the variations in the weight of the core, the weight of the poured molten metal, the cooling time, the constituents of the sand, and to provide uniform foundry sand to a line for molding. A process is disclosed for preparing foundry sand wherein sand is returned per mold after pouring molten metal and wherein the sand is delivered to a line for molding through a line for preparing the sand. The process comprises the steps of obtaining information for classifying the sand Isc, which includes the constituents of the sand, by using information on a pattern Im; using the information for classifying the sand Isc to identify each batch of the returned foundry sand per mold to beformed; storing the returned foundry sand, for which there is the information for classifying the sand Isc, per mold in a storage room for returned foundry sand; and taking out the returned foundry sand, for which there is the information for classifying the sand Isc, per mold from the storage room for returned foundry sand to mull the sand.