An image processing apparatus capable of realizing accurate anti-aliasing with a small memory, without being affected by the order of drawing, and without inducing a drop in the drawing speed, including an anti-aliasing system obtaining edge information from an image after drawing, determining a processing content necessary for the anti-aliasing, and performing the determined processing. Specifically, either of the information of a z-buffer and the information of the normal vector at each pixel obtained at the time of drawing, or both information, is scanned or by the information of normal vectors restored from the information of the z-buffer is used, a state machine for holding the state and a counter for measuring the continuity of an edge are prescribed, the value of which pixel adjacent in which direction to each pixel on each edge and what kind of ratio to use for blending are determined, and the determined values are used for blending. This is performed successively until the pixel values are updated.