The present invention is a method for balancing voltages of a serially connected plurality of cells in a battery pack provided with the plurality of cells. In this method, the voltages are balanced by: connecting a pair of a resistor and a switch, which are serially connected with each other, in a parallel relationship to one of the plurality of cells; switching the switch from an open state to a closed state when a voltage of the one cell increases to a prescribed balance operation starting voltage; and maintaining a state where the cell is charged by a voltage of the same magnitude as a voltage that is applied to both ends of the pair after switching of the switch to the closed state.