Based on an area which was accessed by a just-previous read command and an
area which is required by a present read command, the direction of the access,
the interval between the areas, and the area size are detected, and the position
and size of an area on a disk memory medium where prereading of data is to be carried
out are determined by using the detected values. Therefore, prereading of data
can be efficiently carried out in response to continuous read commands which request
data that are located in the backward direction, i.e., the direction in which an
address decreases, or data that are located separately at equal intervals.