1. A method of generating a dither mask ( 10 ) which is used for conversion of a continuous-tone image ( 100 ) into a halftone image ( 200 ), said method comprising the steps of: a preparation step (S 1 ) for defining a pixel array (M) having a size in accordance with a dither mask to be generated and partitioning said pixel array into a plurality of blocks (B(u, v)); an initial pixel value determination step (S 2 ) for determining an initial pixel value (f(i, j)) for respective pixels which are members of said pixel array, in a manner such that frequencies of occurrence of pixel values within a predetermined numerical value range that is required by said dither mask become substantially uniform in each individual block; a function value computation step (S 3 ) for selecting a plurality of exchange target pixels at random from among pixels in a single block and computing function values of a predetermined evaluation function that indicates a characteristic of a pixel value configuration of pixels in said single block for two cases of performing exchange of pixel values among said plurality of exchange target pixels and not performing said exchange; an exchange judgment step (S 4, S 5, S 6 ) for judging whether or not the exchange of pixel values among said plurality of exchange target pixels should be performed by comparing said function values for the two cases; and a pixel value exchange step (S 7 ) for performing a process of the exchange of pixel values among said plurality of exchange target pixels, when it has been judged in said exchange judgment step that the exchange should be performed; wherein said function value computation step, said exchange judgment step and said pixel value exchange step are executed repeatedly for all blocks until a predetermined condition is met (S 8, S 9, S 10 ) and the dither mask is generated by pixel values of said pixel array that is finally obtained.