1. The related-word registration device comprising: a receiving means that receives a search query of a search word entered by the user; a search query storing means that stores the received search queries in accordance with reception order; a search query extracting means that extracts, from the search query storing means, a preceding search query whose reception order is earlier than that of the received search query on the basis of a preset search query extracting condition; a character string set storing means that stores, as a character string set, a preceding search word constructing the extracted preceding search query and a search word constructing the received search query; a character string extracting means that extracts a character string set having the search word which is the same or similar to the preceding search word from the character string set storing means in accordance with a preset character string set extraction start condition; a related-word specifying means that specifies a character set as a related word from the extracted character string set on the basis of a preset registration condition; and a related-word registering means that registers the specified character string set as related words into a related-word database.