A related-word registration device comprising: a receiving means configured to receive a search query of a search word entered by the user; a search query storing means configured to store the received search queries in accordance with reception order; a search query extracting means configured to extract, 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, wherein the search query extracting means is configured to extract a preceding search query received within predetermined time since received time of the search query as the search query extraction condition; a character string set storing means configured to store, as a character string set, a preceding search word of the extracted preceding search query and a search word of the received search query; a character string extracting means configured to extract character string sets having the same or similar preceding search words from the character string set storing means in accordance with a preset character string set extraction start condition, wherein the character string extracting means is configured to extract the character string set in the case where the number of search queries in the search query storing means or the total number of character string set groups exceeds a predetermined threshold as a character string set extraction start condition; a related-word specifying means configured to specify a character string set as related words from the extracted character string sets on the basis of a preset registration condition, wherein the preset registration condition is satisfied in the case where the number or the proportion of character string sets having the same or similar search words among the extracted character string sets exceeds a predetermined threshold; and a related-word registering means configured to register the specified character string set as related words into a related-word database, wherein search words, respectively preceding search words, are similar to each other when the value of similarity based on the distance between their character strings is equal or larger than a predetermined threshold. a receiving means configured to receive a search query of a search word entered by the user; a search query storing means configured to store the received search queries in accordance with reception order; a search query extracting means configured to extract, 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, wherein the search query extracting means is configured to extract a preceding search query received within predetermined time since received time of the search query as the search query extraction condition; a character string set storing means configured to store, as a character string set, a preceding search word of the extracted preceding search query and a search word of the received search query; a character string extracting means configured to extract character string sets having the same or similar preceding search words from the character string set storing means in accordance with a preset character string set extraction start condition, wherein the character string extracting means is configured to extract the character string set in the case where the number of search queries in the search query storing means or the total number of character string set groups exceeds a predetermined threshold as a character string set extraction start condition; a related-word specifying means configured to specify a character string set as related words from the extracted character string sets on the basis of a preset registration condition, wherein the preset registration condition is satisfied in the case where the number or the proportion of character string sets having the same or similar search words among the extracted character string sets exceeds a predetermined threshold; and a related-word registering means configured to register the specified character string set as related words into a related-word database, wherein search words, respectively preceding search words, are similar to each other when the value of similarity based on the distance between their character strings is equal or larger than a predetermined threshold.