1. A text mining system comprising: an analysis target data pair search processor which judges whether there is a commonality in expressions among pieces of text data, said pieces of text data being included in plural pieces of analysis target data, respectively; an analysis viewpoint generation processor which generates a plurality of common analysis viewpoints which is a condition to extract an expression from each of said pieces of analysis target data with said commonality; a positive example set identification processor which identifies, for each of said plurality of common analysis viewpoints, one or more positive example sets each including an expression matching a corresponding one of said plurality of common analysis viewpoints in each of said pieces of analysis target data with said commonality; a characteristic quantity calculation processor which calculates, for each of said plurality of common analysis viewpoints, characteristic quantity showing a degree of characteristic expression of said one or more positive example sets for each of expressions in each of said pieces of analysis target data with said commonality; and a characteristic expression ranking processor which provides, for each of said plurality of common analysis viewpoints, ranks for said expressions in descending order of said calculated characteristic quantity, wherein said analysis target data pair search processor extracts a common analysis viewpoint for said pieces of analysis target data with said commonality from said plurality of common analysis viewpoints in case that a difference in ranks provided for each of said expressions is equal to or greater than a predetermined threshold for said common analysis viewpoint.