PURPOSE: To shorten data processing time by classifying an allowable observation direction into finite numbers of observation direction aggregation and discriminating the aggregation from the bordering with the aggregation of boundary surfaces by each aggregation in an observation direction space.

CONSTITUTION: The number of necessary expressed order list is different according to geometrical shapes of objects. The number of conceptual observation axis 43 or the number of list to be prepared is designated by an operator. For instance, eight observation axes are designated. Therefore, a sphere (directional space) 41 is divided into eight sections. The conceptual observation axis 43 is one of eight observation axes and demarcates a section 44 bordering with a horizontal division surface 45, a first vertical division surface 46 and a second vertical division surface 97. A section to be noted is demarcated not by a visual field cone but by the intersection between planes. Therefore, data is easy to be worked and processing time can be shortened by defining a concern direction in this way.