[PROBLEMS] To provide a false alteration detector wherein watermark information is automatically embedded, an output image is created, watermark information is extracted from the output image, and false alteration on the output image is automatically detected. [MEANS FOR SOLVING PROBLEMS] A false alteration detector computes an output value by applying a feature extracting filter (40) very reactive to a specific shape in judgment areas (32) of an original image, divides each judgment area into judgment blocks, computes an evaluation value of each judgment block using the output value, embeds as watermark information (28) the position information on the judgment area or the evaluation value of each judgment block into the original image so as to create a watermarked output image, extracts the watermark information, creates a comparison original image, computes a comparison output value by applying a feature extracting filter in the judgment area of the comparison original image using the position image information on the judgment area, divides the judgment area of the comparison original image into judgment blocks, computes a comparison evaluation value, and judges whether the judgment area of the comparison original image is falsely altered or not from the differences between the evaluation values of each judgment block of the original image and the comparison evaluation values of each judgment block.
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