1. An information extraction system that extracts phrases in documents from the documents and outputs the extracted phrases, comprising: an input unit that receives an input word list including a plurality of phrases; a pattern determining unit that selects one document from a plurality of documents including documents whose formats are different from each other, and finds a pattern, which separates a phrase included in an input word list from the other word in the selected document, as a pattern, for each of the documents; a phrase candidate extracting unit that extracts a character string separated by a pattern from a document used in determining the pattern and determines the character string as a candidate of a phrase; and a phrase selecting unit that selects a candidate of a phrase or a partial character string satisfying a predetermined condition as a phrase of an output subject among candidates of phrases extracted by the phrase candidate extracting unit or partial character strings included in the candidates of the phrases.