1. A method of determining a threshold arrangement for generating a gradation image to determine a position of at least one threshold of the same value for a next gradation when positions of thresholds ranging from a smaller threshold to a threshold for a given gradation are determined in a threshold arrangement, comprising the steps of: (A) determining at least one candidate position for the position of at least one threshold of the same value for the next gradation (step S 5 ); and (B) determining the position of the threshold for the next gradation from said candidate position (steps S 6 - S 9 ) said step (B) comprising the steps of: extracting a low-frequency component from image data obtained based on the threshold arrangement in which the positions of the thresholds ranging to said threshold for the given gradation are determined (1st step; step S 7 ); determining a low-frequency component intensity at said at least one candidate position (2nd step; step S 8 ); determining the candidate position where the determined low-frequency component intensity is weakest as the position of the threshold for the next gradation (3rd step; step S 9 ); and repeating said step of extracting a low-frequency component, said step of determining a low-frequency component intensity, and said step of determining the candidate position until all positions of at least one threshold of the same value for the next gradation are determined.