A noise shaping processing part performs noise shaping processing on pixel data. An adding circuit performs accumulative adding processing on the noise-shaped pixel data. A first bit-shift part performs bit-shift processing on the accumulatively-added and noise-shaped pixel data. A second bit-shift part performs bit-shift processing on unprocessed pixel data. An inter-pixel subtracting circuit calculates a gradation level difference between neighboring current pixel and previous pixel in a horizontal direction in the unprocessed pixel data. A boundary judging circuit judges presence of a boundary between a low-gradation area and a high-gradation area in the horizontal direction of the unprocessed pixel data, based on a comparison between the gradation level difference and a prescribed threshold value. A selecting circuit selects an output of the first bit-shift part when the boundary judging circuit judges there is no boundary, and selects an output of the second bit-shift part when judged there is a boundary.