To provide a calculation method for obtaining distortion distribution required to shape plate material to objective curved surface, while giving a shape when developing a curved surface to a plane.

This method consists of following processes; a. a process which converts a curved surface shape displayed by coordinate of discrete points etc. into a function, b. a process which draws geodesic lines in longitudinal and lateral directions on a curved surface made by the function, c. a process which performs calculation of distribution of the distortion quantity concerning an undevelopable surface formation necessary for curved surface formation from shape change on a curved surface of a domain surrounded by four geodesic lines forming a square on a plane or change of interval on the geodesic lines which satisfies parallel condition on a plane, and d. a process which extracts developable transformation component bringing the difference when difference exists between the undevelopable surface obtained by giving distribution of distortion quantity obtained by each process above mentioned to the plate material and the objective curved surface, and which performs calculation of both of the undevelopable transformation and developable transformation.