To provide a mold which can prevent production of a blowhole in a cast product to be molded into a product cavity by a simple device, and to provide a casting method using the mold and a method for designing the mold.

The mold 1 includes: a product cavity 2 in which a molten metal 51 is filled to mold a product 52; a supply cavity 3 which is connected to one side of the product cavity 2 and supplies the molten metal 51 filled from a mold inlet 31 to the product cavity 2; and a cooling cavity 4 which is connected to the other side of the product cavity 2 and cools the molten metal 51 filled therein before the product cavity 2 and the supply cavity 3 are cooled. Modulus M (=V/S) of the cavities 2, 3, 4 being the ratio of volume V to cooling surface area S satisfy the relationship of Ms>Mp>Mc where Mp denotes the modulus of the product cavity 2, Ms denotes the modulus of the supply cavity 3, and Mc denotes the modulus of the cooling cavity 4.