1. An expression detecting system for detecting preference expressions indicating evaluators' likes and dislikes of a specific object from texts describing the evaluations of said specific object, comprising: a text storing part for storing each of said texts describing the evaluations of a specific object in association with an attribute of the text; an expression extracting part for extracting an evaluating expression describing evaluations of said specific object from each of said texts; a polarity determining part for determining whether the evaluating expression extracted by said expression extracting part has a positive polarity or a negative polarity, wherein said positive polarity indicates a favorable evaluation of said specific object and said negative polarity indicates an unfavorable evaluation of said specific object; an attribute inputting part for inputting an attribute of a text that is designated as an object for detecting said preference expressions; and a preference expression detecting part for detecting an evaluating expression, which is detected from a text having an attribute input by said attribute inputting part, from evaluating expressions extracted by said expression extracting part as a preference expression, and outputting the preference expressions in association with a frequency of the preference expressions being determined as a positive polarity or a negative polarity in the text having the attribute; said detecting said evaluating expression responsive to attributes including age group, sex or residential area from the evaluating expressions detected from the text; wherein said preference expression detecting part is implemented on a computer having a story memory; and a certainty factor storing part for storing certainty factor useful in determining polarity of an evaluating expression responsive to the certainty factor.