A controller unit, a plurality of storage devices, a log area, and a normal area are provided. A quantity of log storage devices forming the log area is smaller than a quantity of normal storage devices forming the normal area. In an operational period, in which the controller unit may accept an input-output command, a power consumption condition of the log storage devices is constantly in a normal condition in which an input or output of data can be carried out, while a power consumption condition of the normal storage devices is in an energy saving condition in which a power consumption is lower than in the normal condition. The controller unit, when receiving the input-output command if the power consumption condition of the normal storage devices is the normal condition, carries out an input or output with respect to both the normal area and the log area.