The charge capacity of a battery for supplying electric power to a volatile memory and a nonvolatile memory is increased without increasing the power source capacity.A control unit for controlling data input/output processing on a storage device is configured by combining a plurality of units which become load on a power source; and at the time of power interruption of a power source, the control unit saves data and information, which are stored in a volatile memory, to a nonvolatile memory. A battery charged by the power source supplies electric power to the volatile memory and the nonvolatile memory at the time of the power interruption. When the electric power is supplied from the power source to the control unit at the time of power recovery after the power interruption, a main controller selects an activation target unit, which is to be activated, from among the plurality of units belonging to the control unit and controls activation of the selected activation target unit. A battery controller controls a charging current supplied to the battery based on surplus power indicating a difference between total power consumption by the selected activation target unit and a capacity of the power source.