A system, method and computer program product for searching at high speed for documents matching a dependency pattern from document data containing a large volume of text documents. The system includes a storage device for storing, index storage means for storing in the storage device occurrence information, receiving means for receiving information, reading means for reading from the index storage means, and searching means for comparing occurrence information. The method and computer program product include the steps of storing in the storage device, receiving information, reading from the storage device, comparing occurrence information, and searching. The computer program product includes instructions to execute the steps of storing each of the plurality of document data in the storage device, storing in the storage device occurrence information.