1. A system for making a search for a document in accordance with a query of a natural language, comprising: a first interface which accepts a user specified first question sentence represented in a natural language; question analysis means for analyzing the first question sentence accepted by the first interface, the question analysis means generating a plurality of second question sentences for individually querying each of a plurality of matters from the first question sentence when the first question sentence queries the plurality of matters; a search engine configured to make searches for documents which match respective matters queried by the plurality of second question sentences from a morphological index database by index searches according to the plurality of second question sentences generated by the question analysis means, the morphological index database storing morphological analysis results for a plurality of documents as indexes; and a second interface which provides a user with a search result for the first question having reflected thereon results of document searches for the plurality of second question sentences by the search engine.