1. A requirement acquisition system comprising: a phrase extraction section, implemented by a processor, configured to extract, from a document being a group of a plurality of character strings, one or more respective partial string which is a common part of the plurality of character strings as one or more respective important phrase; a phrase storage section, implemented by a processor, which stores the one or more important phrase; a character string extraction section implemented by a processor which is configured, for each of the one or more important phrase, to finish processing when the important phrase does not exist, and to extract a representative character string of the document as a respective candidate character string and delete the respective candidate character string from the document when the important phrase exists; a phrase deletion section implemented by a processor which is configured to delete the respective important phrase of the one or more important phrase following finishing of processing related to the respective important phrase by the character string extraction section; a character string addition section implemented by a processor which is configured, for each respective candidate character string, to set the respective candidate character string as an important character string when the number of the important phrase being deleted is one or more; and an important character string storage section implemented by a processor which is configured to store the important character string.