Acquiring current 3D space data of cities, buildings, and rooms rapidly and in detail has become indispensable. When the point cloud data of an object or space scanned by a 3D laser scanner is converted into polygons, it is an accumulation of small polygons. When object or space is a closed flat plane, it is necessary to merge small polygons to reduce the volume of data, and to convert them into one polygon. When an object or space is a closed flat plane, each normal vector of small polygons theoretically has the same angle. However, in practise, these angles are not the same. Therefore, the purpose of this study is to clarify the variation of the angle of a small polygon group that should become one polygon based on actual data. As a result of experimentation, no small polygons are converted by the point cloud data scanned with the 3D laser scanner even if the group of small polygons is a closed flat plane lying in the same plane. When the standard deviation of the extracted number of polygons is assumed to be less than 100, the variation of the angle of the normal vector is roughly 7 degrees.