In a threshold matrix generation part 3 implemented by a computer, a tint image in a specified gray level is generated from temporary lighting order set in a matrix area, and a smoothing operation is performed on the tint image, to generate a grayscale evaluation image. The lighting order is modified by changing ordinal numbers in the lighting order of respective positions included in specified areas in the matrix area which correspond to areas around boundaries of halftone dot areas in the tint image on the basis of values of corresponding pixels in the evaluation image. With this operation, when the smoothing operation is performed on a tint image in the specified gray level generated in accordance with a modified lighting order, a variation among values of pixels included in the specified areas becomes smaller than a variation among values of pixels included in the specified areas in a case where the smoothing operation is performed on the tint image in the specified gray level generated in accordance with an unmodified lighting order. Then, threshold values are determined in the matrix area with reflecting the modified lighting order, to generate the threshold matrix which is possible to suppress graininess in a generated halftone dot image.