There is provided an image processor for obtaining a correction curve used in a gradation correcting process smoothly and quickly. In the image processor, a curve storing section stores reference correction curves and a correction amount storing section stores correction amounts corresponding to a plurality of specified input density values. A halftone output gradation processing section calculates the correction value to the specified input density value by using the reference correction curve and the set of correction amounts in copying original and calculates a correction value to the remaining input density value by means of interpolation operation for moving one part of the reference correction curve in the direction parallel to the axis of density value and for expanding or compressing in that direction.