1. A related-word registration device comprising: a search query obtaining means that obtains a search query including a search word entered by the user; a search query storing means that stores the obtained search query in association with user identification information and acquisition time of the search query; a search query extracting means, in the case where a predetermined user operation is detected, that extracts search queries corresponding to the user identification information of the user; a first search query specifying means that specifies a first search query from the extracted search queries; a second search query specifying means that specifies a second search query whose acquisition time is earlier than acquisition time of the first search query, from the extracted search queries; and a related-word registering means that registers, as related words, a search word in the first search query and a search word in the second search query into a related-word database.