To suitably blend the necessary amount of a supplemental component into molding sand separated from a cast product from one or a little fixed number of already known kinds of molds at a low cost, in a closed loop-state casting line.

A molding flask is circulated with a closed loop state molding flask circulating device and the empty molding flask is conveyed into a molding device at a molding stop position and the mold is shaped, and at a molten metal pouring stop position, the molten metal is poured into the mold and the mold already poured with the molten metal is cooled during conveying to the cooling stop position. The mold is conveyed into a mold shake-out device at a molding flask shaking-out stop position and shaken out, and the empty molding flask is returned back into the molding flask circulating device. The cast product is separated from the shaking-out one or few fixed number of known kinds of the molds, and the molding sand is recovered. The necessary amount of supplemental component is blended into this recovered molding sand, and the molding sand blended with the supplemental component is kneaded with a kneader while adjusting the water amount and this kneaded molding sand is conveyed into the molding device and charged into the molding flask, and the mold is shaped.