1. A defacement degree determination apparatus comprising: an image input unit configured to input an image of a printed material; a rotation angle detection unit configured to detect a rotation angle of the printed material with respect to a reference position based on the input image input by the image input unit; an alignment unit configured to align positions of the input image printed using a plurality of printing plates with registered images registered in advance; a target rotation conversion unit configured to apply rotation conversion to a plurality of printed patterns registered in correspondence with the plurality of printing plates in accordance with the rotation angle; a target position detection unit configured to detect, from the input image, a plurality of matching similarity maximum positions where matching similarities with the respective rotated printed patterns become maximum in correspondence with the plurality of printing plates; a matching similarity calculation unit configured to calculate a plurality of matching similarities between the respective rotated printed patterns and the respective matching similarity maximum positions, and a plurality of matching similarities between the respective rotated printed patterns and a plurality of neighboring positions that neighbor the respective matching similarity maximum positions in correspondence with the plurality of printing plates; a function approximation unit configured to approximate, by a function, a gradient of the plurality of matching similarities indicating matching similarity results with respect to an identical rotated printed pattern in correspondence with the plurality of printing plates; and a target true position estimation unit configured to estimate true positions of the respective rotated printed patterns corresponding to maximal values calculated by the approximation by the function in correspondence with the plurality of printing plates, a plate separation processing unit configured to separate the input image into a plurality of printed components corresponding to the plurality of printing plates, and extract defacement features from the respective printed components; and a defacement degree determination unit configured to determine a defacement degree of the printed material based on the defacement features extracted from the respective printed components, wherein the alignment unit aligns the positions of the input image and the registered images based on the true positions of the respective rotated printed patterns estimated by the target true position estimation unit in correspondence with the plurality of printing plates.