1. A document extracting method for extracting specific document data from stored document data, the method comprising steps of: storing a document index, indicating a document including a plurality of pages, in association with document data corresponding to each page included in the document; storing feature data, calculated based on a feature point extracted from document data and indicative of a feature of the document data, in association with the document data; obtaining input document data serving as new document data; extracting a feature point from the obtained input document data; generating feature data indicative of a feature of the input document data based on the extracted feature point; comparing the generated feature data with the stored feature data, thereby judging similarity between the document data associated with the stored feature data, and the input document data; obtaining a document index associated with document data judged as document data highly similar to the input document data; and extracting a plurality of pieces of document data corresponding to a plurality of pages included in a document indicated by the obtained document index.