To guarantee a data transfer rate irrespective of a performance of a memory card.A parameter to show a data transfer efficiency is previously recorded in a recording medium 101 in which data is written per data size that can be increased and decreased stepwise and the data transfer efficiency is variable in accordance with the data size. Next, a data accessing apparatus 105 issues a parameter acquisition command to the recording medium 101. The recording medium 101 which received the parameter acquisition command transmits the parameter. The data accessing apparatus 105 collates the received parameter with the data transfer efficiency required in the data to be written/read to thereby select an optimum data size. Then, the data accessing apparatus 105 writes/reads the data to the recording medium 101 based on the selected optimum data size.