The present invention realize a high speed retrieval performance in a document retrieval system referring to partial data of documents including structured data such as XML documents and electric mails, without providing further memory. The present invention includes storage means for storing documents to be retrieved onto a disk device, a calculation means for calculating an allocated capacity of the memory, and storage means for saving, onto the memory, partial data of the documents stored on the disk device by the calculated allocated capacity of the memory. The present invention also includes a first retrieval means for retrieving partial data stored on the memory, determining means for determining whether or not to retrieve the documents stored on the disk device based on the result from the first retrieval, and a second means for retrieving the documents stored on the disk device based on the result from the above determination.