To efficiently construct a three-dimensional model. A product is divided into two parts of a head portion and a neck portion. A head form 34 and a neck form 36 are selected as forms of a three-dimensional model. The basic dimensions of each part are inputted to calculate dimension values of each part, and a combination of a basic form and a cut part is selected in accordance with a table 38. If it is necessary to change the basic form of the neck form 36, the dimensions of a basic-form model 40 are changed to construct a model 42. Furthermore, a cut part 48 is selected, and the model 42 is processed to construct a model 50. Then, by combining a head-form model 52 and the neck-form model 50, a three-dimensional model 54 of a bolt is constructed.