1. An image data processing method for processing, by a processing unit, image data of an object or a shape by comparing the image data with comparison image data stored in an image information memory to determine a degree of similarity of the object or the shape and the comparison image data, comprising: extracting contour information of an entirety or a part of the object or the shape from the image data; stepwise smoothing the contour information of the entirety or the part of the object or the shape over a plurality of stages to obtain smoothed contour lines on each stage; processing the contour information represented by each smoothed contour line on each stage to divide a contour shape represented by the contour information into one or a plurality of divided curves at each inflection point of each smoothed contour line by calculating a degree of curvature of each curve to store the degree of the curvature as information of each divided curve in the information memory; and obtaining a feature amount of each divided curve on each of the stages to store the feature amount of the smoothed contour lines in the information memory to compare the feature amount with the corresponding feature amount of the comparison image data and to determine the degree of the similarity of the image data with the comparison image data.