1. A threshold matrix generation device for generating a threshold matrix having a specified size for use of halftone processing of a multi-tone image data using a dithering method, wherein the device sets a ratio of a length along a main scanning direction to a length along a subscanning direction (sub-direction) based on a relation between image reproducibility in the main scanning direction and image reproducibility in the sub-direction of an image processing device for forming an image on a paper based on an image data obtained by the halftone processing, determines spatial frequency characteristics having a domain of an ellipse shape having set major axis and minor axis as peak values, and sets a threshold value of each pixel of the threshold matrix in accordance with the determined spatial frequency characteristics.