A control system for a secondary battery configured to exchange electric power with a load, said secondary battery including first and second electrodes (12, 15) each configured to include an active material containing a predetermined material, and an ionic conductor for conducting said predetermined material which is ionized between said first and second electrodes, the control system comprising: a concentration estimating unit configured to estimate an electrolyte ion concentration in an electrolytic solution of said ionic conductor based on a use state of said secondary battery; and a charging/discharging control unit for controlling charging/discharging of said secondary battery based on an estimated value (B#) of said electrolyte ion concentration provided by said concentration estimating unit, so as to maintain said electrolyte ion concentration within a normal range, characterized in that said concentration estimating unit includes a first change ratio estimating unit configured to calculate an estimated value (ΔB) of a ratio of change in said electrolyte ion concentration in accordance with a previously found characteristic regarding a change in said electrolyte ion concentration relative to a charging/discharging current (Ib) and a charging/discharging time (that) in charging/discharging said secondary battery with a fixed current, based on said charging/discharging current and said charging/discharging time of said secondary battery with said charging/discharging current, and a concentration estimated value calculating unit configured to sequentially obtain said estimated value (B#) of said electrolyte ion concentration by accumulating a change in said electrolyte ion concentration caused by use of said secondary battery, according to said estimated value of said ratio of change calculated by said first change ratio estimating unit. first and second electrodes (12, 15) each configured to include an active material containing a predetermined material, and an ionic conductor for conducting said predetermined material which is ionized between said first and second electrodes, a concentration estimating unit configured to estimate an electrolyte ion concentration in an electrolytic solution of said ionic conductor based on a use state of said secondary battery; and a charging/discharging control unit for controlling charging/discharging of said secondary battery based on an estimated value (B#) of said electrolyte ion concentration provided by said concentration estimating unit, so as to maintain said electrolyte ion concentration within a normal range, characterized in that said concentration estimating unit includes a first change ratio estimating unit configured to calculate an estimated value (ΔB) of a ratio of change in said electrolyte ion concentration in accordance with a previously found characteristic regarding a change in said electrolyte ion concentration relative to a charging/discharging current (Ib) and a charging/discharging time (that) in charging/discharging said secondary battery with a fixed current, based on said charging/discharging current and said charging/discharging time of said secondary battery with said charging/discharging current, and a concentration estimated value calculating unit configured to sequentially obtain said estimated value (B#) of said electrolyte ion concentration by accumulating a change in said electrolyte ion concentration caused by use of said secondary battery, according to said estimated value of said ratio of change calculated by said first change ratio estimating unit. a first change ratio estimating unit configured to calculate an estimated value (ΔB) of a ratio of change in said electrolyte ion concentration in accordance with a previously found characteristic regarding a change in said electrolyte ion concentration relative to a charging/discharging current (Ib) and a charging/discharging time (that) in charging/discharging said secondary battery with a fixed current, based on said charging/discharging current and said charging/discharging time of said secondary battery with said charging/discharging current, and a concentration estimated value calculating unit configured to sequentially obtain said estimated value (B#) of said electrolyte ion concentration by accumulating a change in said electrolyte ion concentration caused by use of said secondary battery, according to said estimated value of said ratio of change calculated by said first change ratio estimating unit.