In processing to thicken a white thin line, the application range thereof is controlled to as to prevent a white thin line not intended by a user from being thickened. The thickening processing is performed for a line that has a density less than or equal to a predetermined density and includes a pixel having attribute information of a drawing object; and not performed for a line that has a density less than or equal to the predetermined density and includes a pixel not having attribute information of the drawing object.