PURPOSE: To prevent a face from being missed when two faces vertically exist in one scan line adjacent to the right side of the edge of the face having high priority.

CONSTITUTION: An update logic part 13b is provided to calculate the intersection of an edge and a scan line according to the position coordinate of the edge for each scan line and to set a half edge flag when there is the edge starting/ ending on the scan line, an order logic part 13c is provided to rearrange the intersection coordinates of the respective above-mentioned edges in the direction of scanning, and a disappearing processing correction part 13d is provided to extract only the valid edge, to make the outside edge of an object having the high priority preferential to the object of the low priority entering the back side and to define all the edge data setting the half edge flag on the back face as the object of a disappearing processing when the object of the high priority is completely processed.