1. An image processing apparatus comprising: an image obtaining unit including a plurality of image obtainment devices and configured to obtain image data of a document image; an extraction unit configured to extract a plurality of feature points of the document image; an association unit configured to associate coordinates of a plurality of feature points of a registered document image registered in advance with coordinates of the plurality of extracted feature points of the document image; a correspondence relationship obtaining unit configured to obtain a coordinate correspondence relationship between the coordinates of the plurality of feature points of the registered document image and the coordinates of the plurality of feature points of the obtained document image which are associated with each other; and an angle obtaining unit configured to obtain a skew angle of the document image with respect to a predetermined direction and an assembling angle of the image obtaining unit with respect to a predetermined assembling direction using the obtained coordinate correspondence relationship, wherein the coordinate correspondence relationship is a coordinate transformation coefficient, and the correspondence relationship obtaining unit separately obtains a coordinate transformation coefficient regarding the skew angle of the document image and a coordinate transformation coefficient regarding the assembling angle of the image obtaining unit.