1. A method for extracting a surface from three-dimensional shape data, comprising: setting a standard surface or a standard line with respect to three-dimensional shape data comprising polygons representing an object in a space where the three-dimensional shape data exist; extracting from the three-dimensional shape data polygons each having an inclination angle in a predetermined range with respect to the set standard surface or the set standard line; and setting a collection of the extracted polygons as an extracted surface.