To enhance the roundness or cylindricalness of a product having an axisymmetric shape or the like by enhancing the molding accuracy of an arcuate part.

A rapid prototyping apparatus forms a product having a predetermined shape by controlling the position of a laser irradiator 4. The laser irradiation is arranged in a state movable so as to relatively approach and separate from a table 7 supporting a shaped article in an z-axis direction, on the basis of three-dimensional coordinates data obtained by the coordinates transformation of the STL data of the shaped article formed on the basis of three-dimensional data, as a mean for the coordinates transformation of the STL data. A cylindrical coordinates transformation means for transforming the STL data to cylindrical coordinates data is provided.