To efficiently perform a retrieval processing intended by a user even in the case that a word provided with polysemy is a key word and in the case that the word provided with many synonyms is the key word and to evade excessive retrieval.

A retrieval condition expressed by a retrieval formula, a retrieval sentence or a key word group for retrieval is inputted (S301) and the key word for constituting the retrieval condition is extracted (S302). In the case that the plural extracted key words are present (S303), after obtaining the synonyms for the respective plural extracted key words (S304), the synonym of low co-occurrence is excluded from the obtained synonyms based on co- occurrence between the words (S305) and the key word is extended by using the remaining synonyms (S306). In the case that the plural extracted key words are not present (S303), the synonyms of the extracted key word are obtained (S307) and the key word is extended by using the obtained synonyms (S306).