An .alpha. value in a desired shape in a desired position can be rewritten to a desired value in a desired range. .alpha. value rewriting units for rewriting an .alpha. value showing an opaqueness degree of an image are provided. When the desired position comes, an area setting signal is outputted in a desired shape from an area setting signal generating unit. While the area setting signal is being outputted, in the .alpha. value rewriting unit, an .alpha. value of inputted image data is rewritten to a desired value. The rewriting of the .alpha. value can be dynamically realized independent of software. By preparing a quadrangular area calculating unit and a circular area calculating unit as an area setting signal generating unit, a quadrangular area and a circular area can be set by hardware.