An image processor for converts input grayscale values into output grayscale values having two or more grayscales by dividing an input image into pixel groups each including pixels and applying a dither matrix to each pixel group a center of gravity position of the pixel group from the input grayscale value of each pixel included in the pixel group; the center of the dither matrix on the center of gravity position of the pixel group; and output the output grayscale values by corresponding the positioned dither matrix and the input grayscale value of each pixel included in the pixel group. By this, the center of the dither matrix is positioned on the center of gravity position of the pixel group. So dots are generated at the most appropriate positions to reproduce an input image faithfully.