An image data generation device, comprising: an image data acquisition means for acquiring image data; a division means for spatially dividing the acquired data into a plurality of matrices; a priority order setting means for setting a priority order for performing gradation conversion of picture elements that make up the image data that corresponds to each of the matrices, and setting the priority order so that the priority order becomes lower going from picture elements located in the center of the matrix toward picture elements that are located on the outside edges of the matrix; a gradation conversion means for performing gradation conversion of the picture elements according to the priority order; and an image data generation means for generating image data for printing based on a dot pattern that is formed by the picture elements after the gradation conversion, wherein the gradation conversion means performs gradation conversion of the picture elements according to a gradation level of 3 or more, characterised in that the gradation conversion means further performs the gradation conversion on adjacent picture elements, being picture elements that are adjacent to core picture elements that have that maximum gradation values among all of the picture elements after the gradation conversion so that the gradation values of the adjacent picture elements become an intermediate gradation value, wherein the number of adjacent picture elements is determined as a fringe width and high priority order adjacent picture elements out of the fringe width become the maximum gradation value as core picture elements. an image data acquisition means for acquiring image data; a division means for spatially dividing the acquired data into a plurality of matrices; a priority order setting means for setting a priority order for performing gradation conversion of picture elements that make up the image data that corresponds to each of the matrices, and setting the priority order so that the priority order becomes lower going from picture elements located in the center of the matrix toward picture elements that are located on the outside edges of the matrix; a gradation conversion means for performing gradation conversion of the picture elements according to the priority order; and an image data generation means for generating image data for printing based on a dot pattern that is formed by the picture elements after the gradation conversion, wherein the gradation conversion means performs gradation conversion of the picture elements according to a gradation level of 3 or more, characterised in that the gradation conversion means further performs the gradation conversion on adjacent picture elements, being picture elements that are adjacent to core picture elements that have that maximum gradation values among all of the picture elements after the gradation conversion so that the gradation values of the adjacent picture elements become an intermediate gradation value, wherein the number of adjacent picture elements is determined as a fringe width and high priority order adjacent picture elements out of the fringe width become the maximum gradation value as core picture elements.