1. A threshold matrix generation method for generating a threshold matrix which is compared with a grayscale original image in generating a halftone dot image representing said original image, comprising the steps of: a) setting ordinal numbers in lighting order to respective positions within a matrix area in which a threshold matrix is generated so that connecting of dots of halftone dot areas occurs at random with increase of gray level within a predetermined range, said lighting order indicating an order for including pixels in said halftone dot areas with increase of gray level; b) binarizing said ordinal numbers in said matrix area on the basis of a specified gray level within said predetermined range of gray level to generate a tint image; c) generating a grayscale evaluation image by performing a smoothing operation on said tint image; d) modifying said lighting order by changing ordinal numbers of positions included in at least specified areas which correspond to areas around boundaries of halftone dot areas in said tint image on the basis of values of corresponding pixels in said evaluation image; and e) determining threshold values in said matrix area with reflecting a modified lighting order, wherein a variation among values of pixels included in said specified areas in a grayscale image which is generated by performing steps b) and c) on ordinal numbers in said modified lighting order is smaller than a variation among values of pixels included in said specified areas in a grayscale image which is generated by performing steps b) and c) on ordinal numbers in said lighting order which is set in said step a).