Object image data of a processing object image, which virtually has cells each including a set of M×N pixels, is processed. The cells are arranged in steps and are shifted by Dx pixels and by Dy pixels. An input raster buffer stores (N−1) raster data including pixel values of the processing object image. A data updating circuit updates the raster with inputted pixel values. Cell buffers store M×N pixel values regarding one of the cells. Processing circuits output partial data for the processed image data by ubmining a center of gravity position of gradation values in the one of the cells from the M×N pixel values, A control circuit controls storing pixel values regarding each of cells in k-th step using the raster data inputted and the raster data stored when “k•(N−1)+1”-th raster data is inputted to the data updating circuit.