To provide a document retrieval device, method and program for making it possible to process registration and deletion at high speed and to calculate scores of retrieval relevance ratios.

The document retrieval device has a document data file 410; a plurality of transfer files 430; a document length file 420; a text splitting part 340 for splitting document data 410b stored in the document data file 410 into index units 430a; and a registering process part 310 for determining the character-string position on the character string of the document data of an index unit 430a split by the text splitting part 340, and registering the character-string position in either one of the plurality of transfer files 430 as the appearance position 430b of the index unit 430a on the document data while associating the position with a document identification number 410a and the index unit 430a.