1. A denoising method comprising: at least one processor for implementing the following steps: generating, based on a digital image acquired by imaging a subject, a plurality of band-limited images respectively representing components of a plurality of frequency bands of said digital image; obtaining a plurality of converted images by performing on respective pixel values of said band-limited images a nonlinear conversion process, in which an absolute value of an output value is reduced to an absolute value of an input value or less, and in which, for an input value whose absolute value is a predetermined threshold value or less, an absolute value of an output value becomes greater as an absolute value of said input value becomes greater and, for an input value whose absolute value is greater than said predetermined value, an absolute value of an output value becomes less than or equal to an absolute value of an output value corresponding to said predetermined threshold value; multiplying the pixel values of said converted images by a predetermined subtraction coefficient; and obtaining a pixel value of a processed image by subtracting the pixel values of said converted images multiplied by said predetermined subtraction coefficient, from a pixel value of said digital image, wherein the predetermined subtraction coefficient is determined according to a pixel value of said digital image so that the subtraction coefficient becomes smaller as the pixel value becomes greater.