A retrieval apparatus 100 for searching document data comprises a document storage area 141 for storing documents to be searched and a document management table 142 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 100 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 t1 taken for a retrieval process by the index scan method and an estimated time t2 taken for the retrieval process by the text scan method, based on the retrieval document size. The retrieval apparatus 100 compares the estimated times t1 and t2, and decides which method to use for a retrieval process, the index scan method or the text scan method.