Plural points present on a boundary surface dividing a subject are set as input points. A plane approximating the boundary surface is determined as a reference plane by using the input points. Plural supplementary points are set on the reference plane on the outside of the subject. A surface approximating the boundary surface is generated by interpolating points between the input points by using the supplementary points and the input points. The subject is divided into separate regions by using the generated surface as a boundary surface.
