An input/output control unit (96) receives two temperature values (T1 and T2) from first and second temperature sensors (13 and 14), respectively, and calculates a temperature difference (ΔT) that is an absolute value of a difference between the two temperature values (T1 and T2). The input/output control unit (96) stores a map determining a relationship between temperature values and target values of input powers (or output powers) of the first and second batteries (11 and 12) required for keeping the temperature difference (ΔT). Based on this map and the two temperature values (T1 and T2) received from the first and second temperature sensors, respectively, the input/output control unit (96) determines the target values of the respective input powers (output powers) of the first and second batteries (11 and 12). A hybrid control unit (92) controls an inverter unit (20) and a booster converter (15) such that the values of the powers input/output to or from the first and second batteries (11 and 12) attain the target values, respectively.