To provide a curved surface interpolation method capable of generating a curved surface which can be beautifully interpolated into an N-sided polygon area by generating one B-spline curved surface covering the N-sided polygon area.

In this curved surface interpolation program for interpolating a curved surface into an N-sided polygon area, four boundary planes indicating the range of the area of a curved surface to be interpolated are decided (S1), and segments going from a plurality of points on each boundary curve of the N-sided polygon area down to the surrounding boundary planes are generated (S2), and the boundary planes to which the segments are belonging are decided (S3), and the boundary curve of the curved surface to be interpolated into the N-sided polygon area is generated from the intersection points of those segments and the boundary planes (S4), and the curved surface to be interpolated is generated from the boundary curve and each point on each segment (S5).