A setup section acquires vertex data including at least homogeneous coordinate and window coordinate relevant to two vertexes of rendering primitive, that is, segment. An edge coefficient calculation unit calculates an edge coefficient used for determining whether or not a pixel exists inside a projection area (parallelogram) of the segment. A bounding box calculation unit calculates a bounding box of the projection area from at least one vertex data of two vertexes and the edge coefficient. A start point determination unit classifies the projection area based on a combination of the edge coefficient, and determines scan start point and scan performing direction of the bounding box in accordance with the classification. An edge function initial value determination unit determines an edge function of the pixel corresponding to the scan start point as an initial value, and carries out a DDA, and thereby, generates setup data for rasterizing.