PURPOSE: To perform another processing even when a disk switching processing is being performed and to simplify the processing of a CPU by separating a task for disk switching.

CONSTITUTION: In the case of generating an ordinary task 30 by receiving an instruction from a host device 20, and also, generating disk switching while performing the processing based on the above task, a task 32 for disk switching is generated, and the task 32 for disk switching is executed preferentially than the ordinary task 30, then, the disk switching processing is performed. In other words, the disk switching processing is separated from the ordinary task 30, and it is processed as another task. In such a manner, it is possible to execute another instruction even when the disk switching processing is being performed, and to simplify the processing as a whole.