To calculate a solid form from a plurality of curved surfaces in a high speed with high accuracy in a three dimension machining method for metallic materials.

A primary corresponding line a (primary corresponding lines b and c likewise)is defined by associating a trim curved line A1 of an input curved surface #A with a corresponding trim line B1 of another curved surface #B, and an identical dot x is set at endpoints a1, b1 and c1 to which the primary corresponding line a, b and c are respectively close. The identical dot x is defined as a vertex and a phase is defined at the vertex, and the vertex is made to correspond as the endpoint of the primary corresponding line. A secondary corresponding line a', b' and c' are generated at the vertex from the primary corresponding lines a, b and c which are identical by the defined phase. The solid form which comprises curved surfaces #A', #B' and #C' by modifying curved surfaces input so as to pass by the secondary corresponding lines a', b' and c' generated as said above.