1. A method for converting a continuous tone image into a pseudo-halftone image, the method comprising the steps of: preparing a threshold matrix which has several elements which are two-dimensionally arranged, each element defining one threshold value receiving data of a continuous tone image desired to be converted into a pseudo-halftone image, the continuous tone image having a plurality of pixels which are two-dimensionally arranged, each pixel having one density value; and subjecting all the pixels of the continuous tone image to an error-distribution binary conversion process using the threshold matrix, to thereby convert the density values of the pixels into binary values while distributing generated errors to neighboring pixels, wherein the threshold matrix is prepared to define the threshold values which are determined according to a blue noise mask producing manner.