A CPU determines whether a generated low-level drawing command is associated with drawing of a gray object and a color value of the gray object is greater than a determination value. If the generated low-level drawing command is associated with drawing of a gray object and the color value of the gray object is greater than the determination value, the CPU executes an overprint drawing process wherein a minimum limit in number of pixels, which causes no defect in a fixing process, is set and a minimum necessary number of pixels are erased (i.e. thinned out).