1. A document ranking system comprising: a content score calculating unit comprising a processor configured to calculate a content score of a first document with respect to each of at least one word contained in the first document; a contribution coefficient determining unit to determine a contribution coefficient of the first document indicative of the first document's contribution value to a second document with respect to a common word included in the first document and the second document; a contribution score calculating unit to calculate a contribution score of the first document based on the contribution coefficient of the first document; a storage device to store the content score and the contribution score; a ranking unit to rank the first document for the at least one word, based on the content score and the contribution score; and an accumulation coefficient calculating unit to calculate an accumulation coefficient of the first document, the accumulation coefficient corresponding to a contribution by the first document to the contribution score of the first document to the second document with respect to the common word, wherein the ranking unit is configured to apply the accumulation coefficient to the content score and the contribution score, or to apply the accumulation coefficient to the contribution score.