An object of the present invention is to suppress fragmentation occurring due to repeated storing and deleting small capacity file data, while effectively storing large capacity real time file data. To achieve this object, according to the present invention, a block relying on a physical storage area, and a continuous plurality of the blocks, constitute a unit. The unit is set at a capacity at which real time processing is guaranteed. Image data or other such real time file data is stored with in units. Further, in a case of data processed in non-real time, the data is stored in blocks. A unit attribute table for managing the unit is arranged, and based on this unit attribute table, a determination is made as to whether or not the unit can be used at the time of storage.