To form a smooth interpolation face by a simple processing based on an outline plotted on the cross-section of a three-dimensional object even when topology is different for each cross-section of the three-dimensional object.

Outline expression data for specifying the outline of a three- dimensional object for each cross-section are inputted in a first step 50. Second- dimensional density distribution is generated for each cross-section in a second step 51. A three-dimensional field is formed based on the generated second- dimensional density distribution in a third step 52. An interpolation face is formed based on the formed three-dimensional field by using a threshold processing.