a capacitor;

a sampling switch, which is formed by an electric switch, electrically connects any one or more battery cells targeted for measurement among the battery cells which make up the battery-cell group to the capacitor so that the capacitor is charged by these battery cells targeted for measurement, and electrically separates the battery cells targeted for measurement from the capacitor after the capacitor is charged;

a measurement section, which measures the actual voltage of the capacitor after the sampling switch electrically separates the capacitor and the battery cells targeted for measurement, and acquires this voltage as a measurement voltage of the battery cells targeted for measurement;

a transfer switch, which is formed by an electric switch, and electrically connects the capacitor and an input terminal of the measurement section after the sampling switch electrically separates the battery cells targeted for measurement and the capacitor; and

an over-voltage prevention switch, which is formed by an electric switch, and grounds the input terminal of the measurement section by turning on immediately before the sampling switch electrically connects the battery cells targeted for measurement and the capacitor,

wherein the measurement section calculates a calculation measurement voltage, which is the hypothetical voltage of the capacitor charged by the battery cells targeted for measurement, using an equivalent circuit, which indicates the inter-terminal capacitance of the sampling switch by replacing the sampling switch with the capacitor, the calculation measurement voltage including a voltage error which occurs due to the inter-terminal capacitance in the equivalent circuit; calculates the ratio of the measurement voltage to this calculation measurement voltage; multiplies the measurement voltage by this ratio; and thereby, corrects the voltage error of the measurement voltage, and

the sampling switch is disposed at a first branch path, which connects the positive electrodes of the battery cells whose numbers are odd in numerical order from an end of the battery-cell group and a first electrode of the capacitor, and is disposed at a second branch path, which connects the positive electrodes of the battery cells whose numbers are even in numerical order from the end of the battery-cell group and a second electrode opposite to the first electrode; and

the measurement section calculates the calculation measurement voltage, using the following expression (A),

Vx′=(C2×Vx+Cssw×total battery-cell voltage×n)/(C2+Cssw×n)  (A)

wherein:

Vx′: the calculation measurement voltage;

C2: the capacitor's capacitance;

Vx: the measurement voltage of the battery cells targeted for measurement;

Cssw: the sampling switch's inter-terminal capacitance;

n: connected node number;

the connected node number n is the total number of the first branch paths if a positive electric charge is stored in the first electrode, and is the total number of the second branch paths if a positive electric charge is stored in the second electrode.