A related-word candidate group (12b) obtained by extracting candidates of a related word on the basis of a predetermined condition from a search query log (12a) is generated (S1 to S4), a search query of a search word entered by the user is received (S10), a partial character string is generated from a character string of the search word (S13), on the basis of the partial character strings, a candidate character string is extracted from the related-word candidate group (S14), a suitability score of the candidate character string is calculated (S16), the candidate character strings are ranked in order of the scores (S17), a reference line L1 of a suitability score for the ranking is generated on the basis of the suitability score and the ranking (S18), a candidate character string whose suitability score is apart from the reference line by a preset threshold or larger is extracted as a registration character string to be registered as a related word (S19), and the extracted registration character string and the search word are registered as related words into the related-word DB 12c (S20).