1. An image-processing method for creating processed image data by applying a spatial-filtering processing to source image data, comprising the steps of: setting a predetermined upper-limit value for a variation amount of said source image data, before performing an image-conversion processing through which said source image data are converted to said processed image data by applying said spatial-filtering processing; and performing said image-conversion processing for said source image data within a range of said variation amount limited by said predetermined upper-limit value.