PURPOSE: To improve the success rate of information retrieval and to shorten the time required for the information retrieval by making an analyzing means generate and output a degenerative character string substituting for a character string by referring to a semantic pattern table which prescribes rules.

CONSTITUTION: A retrieval key derivation system is equipped with the semantic pattern table 13 wherein the rules for degerating a character strings in word units are prescribed, an analysis part 14 which derives a retrieval key for an inputted character string by referring to a dictionary 12 and the table 13, and an output part 15 which outputs the derived character string. Here, the analysis part 14 and analytic dictionary 12 are used to divide the character string inputted from an input part 11 into words. Then semantic classifications are imparted to the respective divided words. Various combination patterns of the semantic classifications corresponding to the respective words are registered in the semantic pattern table 13. The degenerative character string which substitutes for the character string is derived by using those analytic dictionary and semantic pattern table 13.