A CFA pattern is extracted from captured image data for each first unit region. A first altered region is detected from disturbance of the periodicity of the CFA pattern, and the first altered region is an image region in which copying has been performed from image data different from the captured image data to the captured image data. The feature amount of the captured image data is extracted for each second unit region different in size from the first unit region. The feature amounts are compared for each second unit region to detect a second altered region, and the second altered region is an image region in which copying has been performed from the captured image data to the captured image data. Information concerning the first and second altered regions are output as alteration detection results in the captured image data.