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: a solution request sentence set acquisition unit for acquiring, using a positive example solution request pattern representing a positive example of a sentence including the problem evoking expression and a negative example solution request pattern representing an opposite request to the positive example solution request, a sentence set matching the positive example solution request pattern and a sentence set matching the negative example solution request pattern from a corpus respectively as a positive example solution request sentence set and a negative example solution request sentence set, and storing parts, in the acquired positive example solution request sentence set and negative example solution request sentence set, that correspond to the problem evoking expression in the positive example solution request pattern and the negative example solution request pattern, in association with a positive example and a negative example; and an identification information specification unit 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.