To prevent the deterioration of the relevance factor of a desired document and a retrieval document, which is accompanied by the enlargement of a range being a developing object and to make a document retrieval processing efficient at the time of thesaurus-developing an input keyword.

A hierarchy value (Layer ID) in the related word group unit of a registered keyword is given to a thesaurus which a thesaurus management part 120 manages. At the time of retrieval, the keyword is inputted from a keyword input part 10, a developing condition on same hierarchy development is set from a development condition setting part 110. A keyword development processing part 130 develops the inputted keyword within the range of the related word group of the pertinent input keyword in the registered keywords in the thesaurus dictionary when a developing range 1 is set. When a developing range 2 is set, the hierarchy value (Layer ID) of the related word group is referred to and same hierarchy development is executed by setting the inside of the range of the word on the related word group having the same hierarchy value to be the object.