1. A apparatus for making a search for a document in accordance with a query of a natural language, comprising: a first interface unit configured to input a user specified first question sentence represented in a natural language, the first question sentence including a term representing a query; a morphological analysis unit configured to execute morphological analysis for the first question sentence input by the first interface unit, thereby to divide the question sentence on a word by word basis; a question analysis unit configured to analyze the first question sentence, thereby to generate a plurality of second question sentences from the first question sentence, wherein the question analysis unit includes: a first module configured to specify, based on a morphological analysis result by the morphological analysis unit, a first noun that corresponds to a subjective case of the first question sentence and is included in the first question sentence; a second module configured to extract one second noun or a plurality of second nouns, which are included in the first question sentence, other than the first noun from the first question sentence based on the morphological analysis result; and a third module configured to connect the first noun, each of the plurality of second nouns, and the term representing the query included in the first question sentence for said each of the plurality of second nouns when the plurality of second nouns are extracted, thereby to generate the plurality of second question sentences, each including the first noun, at least one of the plurality of second nouns, and the term representing the query, for individually querying each of a plurality of matters, the first noun also corresponding to the subjective case of each of the plurality of second question sentences; 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 unit, the morphological index database storing morphological analysis results for a plurality of documents as indexes; a storage unit which stores tuning information, the tuning information including a concatenation condition for specifying which of documents acquired by the document searches by the search engine and abstracts of the acquired documents are used as search results for the plurality of second question sentences; a concatenation unit configured to select the documents acquired by the document searches by the search engine or the abstracts of the acquired documents as the search results for the plurality of second question sentences to be concatenated with each other in accordance with the tuning information stored in the storage unit, thereby to generate a search result document which represents a search result for the first question sentence by concatenating search results for the plurality of second question sentences by the search engine; and a second interface unit configured to provide a user with the search result document generated by the concatenation unit as a search result for the first question sentence.