1. An information extraction system for extracting identification information for identifying a problem evoking expression which is an expression evoking a problematic situation, the information extraction system comprising: hardware including a processor; a solution request sentence set acquisition unit implemented at least by the hardware and for acquiring, a sentence set matching a positive example solution request pattern representing a positive example of a sentence including the problem evoking expression as a positive example solution request sentence set, from a corpus, and a sentence set matching a negative example solution request pattern representing an opposite request to the positive example solution request as a negative example solution request sentence set, from the corpus, and extracting the problem evoking expression from a sentence in the positive example solution request sentence set and storing the sentence in association with the problem evoking expression, and extracting the problem evoking expression from a sentence in the negative example solution request sentence set and storing the sentence in association with the problem evoking expression; and an identification information specification unit implemented at least by the hardware and for comparing, for each problem evoking expression, constituent elements of sentences included in the positive example solution request sentence set and the negative example solution request sentence set, and specifying a constituent element characterizing the positive example solution request sentence set and a constituent element characterizing the negative example solution request sentence set, respectively as positive example identification information for identifying a positive example of a sentence including the problem evoking expression and negative example identification information for identifying a negative example of a sentence including the problem evoking expression.