1. A keyword extracting device comprising: text data input means for inputting a text, pattern processing means for carrying out matching and replacement of a character string based on a pattern in regular expression or its equivalent, pattern storage means having at least a keyword component pattern representing a character string capable of being a component of a keyword, keyword component extracting means for extracting, as keyword components, all character strings which are matched with the keyword component pattern and are not overlapped with each other by using the pattern processing means for the text, keyword candidate set generating means for generating a keyword candidate set from each keyword component, and keyword output means for outputting each keyword candidate of the keyword candidate set as a keyword.