In an environment in which storages are intensively collected, many unused areas are generated and no storage resources can be efficiently used as a storage pool. The capacity utilization (data capacity) of a storage device (volume) allocated to a computer is obtained and future capacity utilization is estimated from a change in the data capacity. The capacity of the storage device is compared with upper limit securing capacity and lower limit securing capacity values indicating the upper and lower limits of appropriate allocating capacity calculated from this estimated capacity utilization. When the capacity of the storage device is greater than the upper limit securing capacity, a different storage device, having at least the lower limit securing capacity and no more than the upper limit securing capacity, is allocated from the storage pool, and the storage device allocated previously is collected in the storage pool.