An image processing method characterized by comprising the steps of: determining a first bandwidth based on the image size of input original image data; computing a first filter coefficient array of a first filtering process for obtaining frequency characteristics corresponding to a band limitation using the first bandwidth; generating first filtered image data by subjecting the original image data to the first filtering process using the first filter coefficient array; deriving a PSNR (peak signal to noise ratio) of the first filtered image data by a signal to noise ratio (S/N) between the original image data and the first filtered image data and computing 20×log 10 (S/N), and computing an allocation coefficient used for determining an optimum bandwidth, based on the PSNR; determining the optimum bandwidth corresponding to the computed allocation coefficient by referring to an optimum bandwidth determination table which is set in a manner such that the larger the allocation coefficient, the larger the optimum bandwidth chosen; computing an optimum filter coefficient array of a second filtering process for obtaining the frequency characteristics corresponding to a band limitation using the determined optimum bandwidth; and generating optimum filtered image data by subjecting the original image data to the second filtering process using the optimum filter coefficient array, wherein in the step of determining the first bandwidth, the first bandwidth corresponding to the image size of the original image data is determined by referring to a first bandwidth determination table which is set in a manner such that the larger the image size, the smaller the first bandwidth chosen; in the step of determining the optimum bandwidth, when a plurality of the optimum bandwidth determination tables are provided in correspondence to the image size and a target PSNR, the optimum bandwidth determination table, which corresponds to the image size of the original image data and a designated target PSNR, is selected, and the optimum bandwidth corresponding to the computed allocation coefficient is determined by referring to the selected optimum bandwidth determination table; and in the step of computing the allocation coefficient, the allocation coefficient is computed by dividing a predetermined PSNR, which is experimentally estimated considering no band limitation, by the derived PSNR so that the smaller the derived PSNR, the larger the computed allocation coefficient. determining a first bandwidth based on the image size of input original image data; computing a first filter coefficient array of a first filtering process for obtaining frequency characteristics corresponding to a band limitation using the first bandwidth; generating first filtered image data by subjecting the original image data to the first filtering process using the first filter coefficient array; deriving a PSNR (peak signal to noise ratio) of the first filtered image data by a signal to noise ratio (S/N) between the original image data and the first filtered image data and computing 20×log 10 (S/N), and computing an allocation coefficient used for determining an optimum bandwidth, based on the PSNR; determining the optimum bandwidth corresponding to the computed allocation coefficient by referring to an optimum bandwidth determination table which is set in a manner such that the larger the allocation coefficient, the larger the optimum bandwidth chosen; computing an optimum filter coefficient array of a second filtering process for obtaining the frequency characteristics corresponding to a band limitation using the determined optimum bandwidth; and generating optimum filtered image data by subjecting the original image data to the second filtering process using the optimum filter coefficient array, wherein in the step of determining the first bandwidth, the first bandwidth corresponding to the image size of the original image data is determined by referring to a first bandwidth determination table which is set in a manner such that the larger the image size, the smaller the first bandwidth chosen; in the step of determining the optimum bandwidth, when a plurality of the optimum bandwidth determination tables are provided in correspondence to the image size and a target PSNR, the optimum bandwidth determination table, which corresponds to the image size of the original image data and a designated target PSNR, is selected, and the optimum bandwidth corresponding to the computed allocation coefficient is determined by referring to the selected optimum bandwidth determination table; and in the step of computing the allocation coefficient, the allocation coefficient is computed by dividing a predetermined PSNR, which is experimentally estimated considering no band limitation, by the derived PSNR so that the smaller the derived PSNR, the larger the computed allocation coefficient.