A battery-monitoring device that monitors an assembled battery having a single cell group or a plurality of cell groups, each made up with a plurality of battery cells connected in series, comprising: a cell controller that is started up in response to a startup voltage applied thereto and includes a cell-balancing unit that executes cell balancing operation in order to adjust charge levels of the plurality of battery cells during a time period through which the startup voltage remains equal to or greater than a predetermined voltage, wherein the cell controller includes a startup terminal to which the startup voltage is applied; a control unit that applies the startup voltage to the cell controller and controls the cell controller; a first timer unit that includes a passive element capable of accumulating/discharging electric energy and applies the startup voltage to the cell controller for a predetermined length of time to the startup terminal, characterized in that: the predetermined length of time changes in correspondence to a voltage value of the startup voltage applied by the control unit to the startup terminal of the cell controller; the control unit further includes a voltage-setting unit that sets a voltage value for the startup voltage to be applied to the startup terminal of the cell controller; the control unit can be engaged in operation in a low power consumption mode for minimizing power consumption, and the control unit does not apply the startup voltage equal to or greater than the predetermined voltage to the cell controller when the control unit is engaged in operation in the low power consumption mode; and after the control unit shifts into the low power consumption mode, the first timer unit applies the startup voltage equal to or greater than the predetermined voltage to the cell controller for the predetermined length of time. a cell controller that is started up in response to a startup voltage applied thereto and includes a cell-balancing unit that executes cell balancing operation in order to adjust charge levels of the plurality of battery cells during a time period through which the startup voltage remains equal to or greater than a predetermined voltage, wherein the cell controller includes a startup terminal to which the startup voltage is applied; a control unit that applies the startup voltage to the cell controller and controls the cell controller; a first timer unit that includes a passive element capable of accumulating/discharging electric energy and applies the startup voltage to the cell controller for a predetermined length of time to the startup terminal, characterized in that: the predetermined length of time changes in correspondence to a voltage value of the startup voltage applied by the control unit to the startup terminal of the cell controller; the control unit further includes a voltage-setting unit that sets a voltage value for the startup voltage to be applied to the startup terminal of the cell controller; the control unit can be engaged in operation in a low power consumption mode for minimizing power consumption, and the control unit does not apply the startup voltage equal to or greater than the predetermined voltage to the cell controller when the control unit is engaged in operation in the low power consumption mode; and after the control unit shifts into the low power consumption mode, the first timer unit applies the startup voltage equal to or greater than the predetermined voltage to the cell controller for the predetermined length of time. the predetermined length of time changes in correspondence to a voltage value of the startup voltage applied by the control unit to the startup terminal of the cell controller; the control unit further includes a voltage-setting unit that sets a voltage value for the startup voltage to be applied to the startup terminal of the cell controller; the control unit can be engaged in operation in a low power consumption mode for minimizing power consumption, and the control unit does not apply the startup voltage equal to or greater than the predetermined voltage to the cell controller when the control unit is engaged in operation in the low power consumption mode; and after the control unit shifts into the low power consumption mode, the first timer unit applies the startup voltage equal to or greater than the predetermined voltage to the cell controller for the predetermined length of time.