a data output apparatus for writing and reading data on storage memories , especially such storage devices using storage media like an optical disk . the apparatus efficiently reads data when writing and reading data in storage memories using storage media with a low writing and reading rate such as an optical disk . the apparatus comprises : a first buffer 3 that retains the inputted data stream ; writing means 4 for writing on the aforesaid recording medium the data stream held in the first buffer 3 ; second buffer 6 for holding the data stream to output to external device ; reading means 7 for reading on to the second buffer 6 the data stream recorded on the recording medium ; and predicting means 20 for predicting the duration w to consume the data stream held on the second buffer 6 on the basis of the duration needed for presentation of video information contained in the data stream held on the second buffer 6 . that ensures that writing and reading are real - time .