Retrieving device, retrieving method, and retrieving program

A retrieving device includes: a classifying unit that classifies document data made up of a plurality of items on the basis of a degree of matching of a first item in which a combination of one or more symbol strings is described in a database in which the document data is stored; and an extracting unit that extracts a group of keywords included in a second item from pieces of document data in a same group classified by the classifying unit as related words for retrieval. Moreover, when the retrieving device is implemented as a distributed system on a network, it is possible to create related words for a plurality of machines and to store the related word data and to enable a plurality of users to use the distributed system.