1. A method for fitting a grid-patterned surface data onto a portion of a three-dimensional shape data including a plurality of polygons, the method comprising: a first step of detecting the conditions of a plurality of polygons in the three dimensional shape data that is located at a periphery of a surface to be fitted the surface data; a second step of determining conditions of grid-pattern of the surface data on the basis of detected conditions of the plurality of polygons; and a third step of fitting the surface data whose grid-pattern has been determined onto the three dimensional shape data.