Retrieval apparatus, retrieval method and retrieval program

A retrieval apparatus for searching document data comprises a document storage area for storing documents to be searched and a document management table for storing a data size of a document such that the data size is associated with a document ID for identifying the document. The retrieval apparatus reads out from the document management table data sizes of documents to be searched, and calculates a retrieval document size by adding up the read out data sizes, and calculates an estimated time t taken for a retrieval process by the index scan method and an estimated time t taken for the retrieval process by the text scan method, based on the retrieval document size. The retrieval apparatus compares the estimated times t and t, and decides which method to use for a retrieval process, the index scan method or the text scan method.