1. A question answering document retrieval system, implemented by a computer system, which executes document retrieval in accordance with a retrieval request in question form, comprising: retrieval means for retrieving associated documents on the basis of a keyword extracted from a question serving as a retrieval request in question form from a user, the retrieval means extracting descriptions associated with the keyword and contained in the retrieved documents as first summaries, thereby acquiring document retrieval results including a list of the extracted first summaries; question type determination means for determining a question type of the question by analyzing a semantic role of the question; summary extraction means for extracting descriptions corresponding to answers matching the determined question type as second summaries from original-document data which forms documents indicated by the document retrieval results, the original-document data being included in original-document data stored in an original-document database and forming each document as a retrieval target, the summary extraction means acquiring a list of the extracted second summaries; and an interface configured to display the document retrieval results and the list of the second summaries, as answers to the retrieval request from the user, on a display device which can be used by the user, the document retrieval results including the list of the first summaries, wherein: the retrieval means includes first score assignment means for calculating a first score representing a degree of relevance with a keyword extracted from the question for each document when associated documents are retrieved on the basis of the keyword, selection means for selecting associated documents on the basis of the first scores calculated by the first score assignment means, and first extraction means for extracting the first summaries from the documents selected by the selection means; and the summary extraction means includes second score assignment means for calculating a second score representing a degree of relevance with a keyword extracted from the question for each candidate for a description corresponding to an answer matching the determined question type, and second extraction means for extracting corresponding descriptions as the second summaries on the basis of the second scores calculated by the second score assignment means.