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 determines a ratio of a width of an image for defining reproducibility along a main scanning direction to a width of the image for defining reproducibility along a subscanning direction (sub-direction), the image for defining reproducibility 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 based on the widths of the image for defining reproducibility in the main scanning direction and the subscanning direction, and sets a threshold value of each pixel of the threshold matrix in accordance with the determined spatial frequency characteristics.