A document search system for finding, based on a search keyword, a document containing the search keyword from among a document DB in which documents that are to be search targets are accumulated, the document search system comprising: a related word dictionary in which related words of search keywords are gathered; a related word query generation processing part configured to extract from the related word dictionary a related word of a search keyword contained in a first query that has been entered as a main query, and to generate a second query, which is a new query as a sub-query, using the related word; characterized by: a search execution part configured to calculate an increase/decrease between a hit count for search results returned with the first query and a hit count for search results returned with the second query; and a search result display processing part configured to simultaneously display on a display part: the first query; the search results returned with the first query; the hit count for the search results; the second query; the hit count for the search results returned with the second query; and the increase/decrease, wherein if the search execution part determines there is contained in the search results returned with the second query a new search result that is different from the search results returned with the first query, the search result display processing part, in displaying the search results returned with the second query, displays on the display part with the new search result emphasized, and wherein if it is instructed by a user that the first query is replaced by the second query so that the second query becomes the main query, the related word query generation processing part extracts from the related word dictionary a related word of a search keyword contained in the second as the main query and generates a third query as the sub-query, using the related word, and the search execution part and the search result display processing part repeat their own processing for the main query and sub-query, respectively. a related word dictionary in which related words of search keywords are gathered; a related word query generation processing part configured to extract from the related word dictionary a related word of a search keyword contained in a first query that has been entered as a main query, and to generate a second query, which is a new query as a sub-query, using the related word; characterized by: a search execution part configured to calculate an increase/decrease between a hit count for search results returned with the first query and a hit count for search results returned with the second query; and a search result display processing part configured to simultaneously display on a display part: the first query; the search results returned with the first query; the hit count for the search results; the second query; the hit count for the search results returned with the second query; and the increase/decrease, wherein if the search execution part determines there is contained in the search results returned with the second query a new search result that is different from the search results returned with the first query, the search result display processing part, in displaying the search results returned with the second query, displays on the display part with the new search result emphasized, and wherein if it is instructed by a user that the first query is replaced by the second query so that the second query becomes the main query, the related word query generation processing part extracts from the related word dictionary a related word of a search keyword contained in the second as the main query and generates a third query as the sub-query, using the related word, and the search execution part and the search result display processing part repeat their own processing for the main query and sub-query, respectively. a search execution part configured to calculate an increase/decrease between a hit count for search results returned with the first query and a hit count for search results returned with the second query; and a search result display processing part configured to simultaneously display on a display part: the first query; the search results returned with the first query; the hit count for the search results; the second query; the hit count for the search results returned with the second query; and the increase/decrease, wherein if the search execution part determines there is contained in the search results returned with the second query a new search result that is different from the search results returned with the first query, the search result display processing part, in displaying the search results returned with the second query, displays on the display part with the new search result emphasized, and wherein if it is instructed by a user that the first query is replaced by the second query so that the second query becomes the main query, the related word query generation processing part extracts from the related word dictionary a related word of a search keyword contained in the second as the main query and generates a third query as the sub-query, using the related word, and the search execution part and the search result display processing part repeat their own processing for the main query and sub-query, respectively.