To obtain the improvement of mechanical property in a cast product with the miniaturization of crystal grains constituting the cast product by adopting a low-temperature mold having lower than 0°C mold temperature and promoting the solidification of molten metal.

The mold before pouring molten metal, is cooled at ≤-10°C and also, when the molten metal is poured into the mold, while exhausting gas in the inner part of the mold to the outer part, the molten metal is poured into a product-shaped part formed in the mold and solidified to obtain the cast product. Further, a core set to a portion for forming the thick part of the cast product, is cooled at lower temperature by ≥10°C than the other portion in the mold.