An image forming apparatus includes rendering units that refer to rendering data composing partial regions within a page as input data, generates image data by merging the rendered results, and uses the image data as output data. Each rendering unit holds a rendering position of processing that is currently executed by itself, and detects a rendering position of processing that is currently executed by another rendering unit. When rendering for a partial region that is currently processed is completed, a control unit detects a rendering position held in another rendering unit, and changes allotment of a partial region serving as the next rendering target of its rendering unit so as to reduce the difference between time periods until its rendering unit and another rendering unit complete rendering for the next partial region.