To easily execute automatic design of a core forming mold for casting by selecting an optimum pattern from the shape patterns of plural kinds of core prints stored in accordance with the shape data of the parts formed of cores and designing details shape data in accordance therewith.

The two-dimensional or three-dimensional shape data is and the shape data of the parting planes (cavity planes) of an upper die 5 and a lower die 4 are formed and is converted to two-dimensional data at the time of designing the suction port core molds of, for example, four cylinders. The shape of a core print 2 is then patterned to a duplex type or quadruplex type by whether the specific parts, such as water jackets, exist between the respective ports or not, by which the optimum core print patterns are determined. Further, the sizes of the core print 2 and the cavity shapes of the lower die 4 and the upper die 5 are inputted by using the shape data of the cylinder head and the layout of the core mold is determined. As a result, the automatic design of the core mold is executed and the time required for the design is shortened.