1. An edge evaluation method for evaluating probabilities that pixels constituting image data are edges, comprising: a variation calculating step for calculating a variation in pixel value of each pixel relative to adjacent pixels; and an edge degree calculating step for calculating, for one arbitrary pixel, an edge degree which expresses numerically a probability of the one pixel being an edge, based on variations of a peripheral pixel group consisting of the one pixel and surrounding pixels; wherein, when a total of absolute values substituting for the variations of the peripheral pixel group is regarded as a variation range summation; and an absolute value of a total of the variations per se of the peripheral pixel group is regarded as a variation increase and decrease range; the edge degree is a ratio of the variation increase and decrease range to the variation range summation, or a value corresponding to the ratio.