1. An image processing method for converting an inputted image into an image with a relatively smaller dynamic range, the method comprising the steps of: calculating an edge strength per position on an input image; controlling a threshold of an epsilon filter on the basis of the calculated edge strength; filtering the input image by the epsilon filter by use of the threshold controlled in the step of controlling the threshold; and calculating a coefficient for converting a pixel value according to an output value in the step of filtering, and converting the pixel value per pixel by the calculated coefficient.