PURPOSE: To provide sand mold structure, which can reduce residual stress at each part in a product without prolonging the cooling time to a cylinder block after casting in a sand mold for casting an engine cylinder block.

CONSTITUTION: This mold is constituted of a cavity 15 for forming flat planar molding sand holding part connecting runners 5, 6 at the lower part of the cavity for forming skirt part 3 in sand molds 1, 2 casting cylinder head side in the cylinder block upward and oil pan side downward. By this method, the molding sand in a crank case does not drop at the time of taking out the casting from a flask because of the presence of the molding sand holding part and since the whole product is uniformly cooled because inner face of the skirt part is covered with the sand, the residual stress is reduced.