1. An image processing apparatus comprising: input means for reading an original document by scanning a plurality of partial documents of the original document, the original document being defined as a composite of the plurality of partial documents; storage means for storing the respective plurality of partial documents as respective partial document data; and joint-portion processing means for recognizing adjoining edges of the partial document data by extracting features along edges of each partial document data and comparing the features between different partial document data, and for positioning the partial document data by extracting image data that lie in a direction orthogonal to the adjoining edges of the partial document data and aligning the extracted image data that lie in the direction orthogonal to the adjoining edges.