1. A document retrieval apparatus comprising: a document storage unit for storing a document to be searched; a document size storage unit for storing a data size of the document such that the data size is associated with a document ID for identifying the document; a retrieval document size calculation unit for reading out from the document size storage unit the data size associated with the document ID indicating the document to be searched, and calculating a retrieval document size by adding up the read out data size(s); a prospect time calculation unit for calculating a first estimated time taken for a retrieval process by a first retrieval method and a second estimated time taken for the retrieval process by a second retrieval method, based on the retrieval document size; and a retrieval method decision unit for comparing the first and second estimated times and deciding which retrieval method to use for performing the retrieval process, the first method or the second method.