If it is determined in step S6 that the given period has elapsed, the storage controller 10 carries out new provision necessity determination processing (see FIG. 15) including determining whether any SSD 20 is to be newly provided or not (step S7). Subsequently, the storage controller 10 carries out second-order distribution necessity determination processing including determining whether the second-order distribution is to be carried out or not (step S8). The storage controller 10 checks the extent state management table 520 and determines whether the second-order distribution is to be carried out or not, based on the data distribution state 526 of the extent 40, for instance. More specifically, the storage controller 10 determines whether the data distribution state 526 indicating “SECOND ORDER” exists or not, that is, whether any extent 40 indicating the second-order distribution state exists or not. The storage controller 10 determines that the second-order distribution is to be carried out if any extent 40 indicating the second-order distribution state exists, or determines that the second-order distribution is not to be carried out if any extent 40 indicating the second-order distribution state does not exist. If it is determined that the second-order distribution is to be carried out, the storage controller 10 carries out the second-order distribution.