To enable measurement of the voltage of a secondary battery by a voltage-measuring part, of each secondary battery based on a command signal from a control device, and, moreover, to eliminate power consumption, when the voltage of the secondary battery is not measured.

A voltage-measuring part 20 is connected to each secondary battery 11 of a battery pack, in which a plurality of secondary batteries 11 are connected in series. The voltage-measuring part 20 comprises a voltage-measuring means 21 for detecting the voltage of the secondary battery and converting the detected value into a digital signal; and a switching element 22 that is to be controlled for switching between an on-state to supply electric power from the secondary battery 11, to the voltage measuring means 21 and an off-state. The voltage-measuring part 20 also has a first photocoupler 24 that is to be controlled for on/off setting by the control device which outputs a voltage-measuring command to the voltage-measuring part 20, wherein the light receiving part 24a is connected in series to a control terminal of the switching element 22.

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