When a control section (39) turns on a switch (31, 33, 35), a capacitor (37) is connected to a secondary battery (B1) in parallel. Accordingly, voltage between both ends of each capacitor (37, 38) reaches voltage between both polarities of each secondary battery (B1, B2). Thereafter, when the control section (39) turns off the switch (31, 33, 35) and turns on a switch (32, 34, 36), each capacitor (37, 38) is connected to the secondary battery (B2, B3) in parallel. The capacitor (37, 38) is charged/discharged to balance voltage between both polarities of the secondary battery (B1 to B3).