A control unit calculates a first document count indicating the number of documents that are retrieved based on a first keyword, a second document count indicating the number of documents that are retrieved based on a logical AND of the first keyword and a second keyword belonging to one item, and a third document count indicating the number of documents that are retrieved based on a logical AND of the first keyword and a third keyword belonging to the one item, from the set of documents. The control unit calculates an evaluation value of the one item, based on the first, second, and third document counts. When the evaluation value satisfies a predetermined condition, the control unit outputs recommendation information recommending use of the one item to narrow down documents that are retrieved.