Abstract: This is provided an ingot mold formed of a cast iron consisting essentially of 3.7 to 4.0% C, not more than 1.6% Si, 0.40 to 0.80% Mn, 0.10 to 0.45% P, not more than 0.10% S, 0.020 to 0.050% Mg, the balance Fe with normally appearing impurities. The elements are adjusted to provide a specific carbon equivalent in the range of 3.2 to 3.6% calculated as C.sub.eqv =%C+0.65% Si+0.35% P-35% Mg. In addition, the ingot mold structure of the present invention contains an amount of not more than 5% by volume of carbide, an amount of not more than 25% by volume of ferrite and at least two-thirds of the total graphite volume being spheroidal graphite with the balance pearlite.