According to an embodiment, virtual logical units are each provided with correlative data between a virtual logical unit number and a real logical unit number. In response to an access request specifying a virtual logical unit number from a host, a correlated real logical unit number is obtained by referring to the correlative data, thereby accessing a real logical unit having the obtained real logical unit number. By changing a real logical unit number correlated to a virtual logical unit number, it is possible to access a plurality of real logical units in a time sharing manner through one virtual logical unit number. Accordingly, it is possible to set the number of accessible logical units to be greater than the number of using logical unit numbers.