A threshold matrix generation device for generating a threshold matrix having a specified size for use in FM-halftone processing of multi-tone image data using a dithering method, comprising: generating, using an image processing device that can form an image on a paper based on an image data obtained by the FM-halftone processing, an image of a thin line running along the main scanning direction of the image processing device and a thin line running along the sub-scanning direction of the image processing device, measuring a first width (d) of the thin line running along the main scanning direction and a second width (c) of the thin line running along the sub-scanning direction on the image formed in the image generating step, determining a width ratio of the first width (d) and the second width (c), the width ratio representing a relation between reproducibility of an image in a main scanning direction and in a sub-scanning direction of the image processing device, and generating a threshold matrix by determining a spatial frequency characteristics of the threshold matrix by filtering a Fast Fourier Transform processed initial dot pattern using a band pass filter having an aspect ratio corresponding to said width ratio so that the resulting spatial frequency characteristics has its peak values on an ellipse having a major axis and minor axis, and the ratio of the major and minor axes of the ellipse equals said width ratio, and by setting a threshold value of each pixel of the threshold matrix in accordance with the determined spatial frequency characteristics. generating, using an image processing device that can form an image on a paper based on an image data obtained by the FM-halftone processing, an image of a thin line running along the main scanning direction of the image processing device and a thin line running along the sub-scanning direction of the image processing device, measuring a first width (d) of the thin line running along the main scanning direction and a second width (c) of the thin line running along the sub-scanning direction on the image formed in the image generating step, determining a width ratio of the first width (d) and the second width (c), the width ratio representing a relation between reproducibility of an image in a main scanning direction and in a sub-scanning direction of the image processing device, and generating a threshold matrix by determining a spatial frequency characteristics of the threshold matrix by filtering a Fast Fourier Transform processed initial dot pattern using a band pass filter having an aspect ratio corresponding to said width ratio so that the resulting spatial frequency characteristics has its peak values on an ellipse having a major axis and minor axis, and the ratio of the major and minor axes of the ellipse equals said width ratio, and by setting a threshold value of each pixel of the threshold matrix in accordance with the determined spatial frequency characteristics. the resulting spatial frequency characteristics has its peak values on an ellipse having a major axis and minor axis, and the ratio of the major and minor axes of the ellipse equals said width ratio, and by setting a threshold value of each pixel of the threshold matrix in accordance with the determined spatial frequency characteristics.