The retrieval condition acquisition unit receives a logical operation expression of the keywords as a retrieval condition from a user. The matching document retrieval unit acquires a list of the document IDs corresponding to the keywords inputted, from the word index of the document information storage unit, and applies a specified logical operation to the result to acquire the matching document IDs. The related keyword calculation unit acquires the keywords extracted from the matching documents retrieved by the matching document retrieval unit as the related keywords, and calculates the degrees of relatedness of each of the matching documents. The related part extraction unit accumulates the degrees of relatedness of the related keywords as to each of the matching documents, and extracts the sentences with the appearance orders in the text kept, in the order of the sentence having a higher accumulated value, until the total length of the sentences extracted becomes longer than a predetermined length. The related part output unit displays the sentences acquired by the related part extraction unit as a retrieval result to the user. Thus, the document retrieval device of the invention extracts the related parts of documents that meet the retrieval intention of the user.