PURPOSE: To improve the retrieval efficiency by relieving rationally misreading without eliminating it simply, although the retrieval efficiency drops by that portion, when there is misreading in a part of a keyword constitution character.

CONSTITUTION: In each character of the same rank as a read character-string and a registered keyword, that which coincides above a first prescribed number corresponding to the number of registered keyword characters is selected, and with regard to each selected read character-string thereof, a character set of every rank is derived. In its character set of every rank, when a character which coincides with a character of the corresponding rank of the registered keyword exists above a second prescribed number corresponding to the number of characters belonging to each character set, all of the read character-strings selected previously are regarded as keywords to be retrieved.