To realize a detector, for the module voltage of a set battery, by which the voltage of each battery module of the set battery can be detected with high accuracy by a method wherein the measuring error of a differential amplification-type voltage detection circuit using an operational amplifier is reduced more than that in conventional cases.

The set battery is composed in such a way that one or a plurality of battery blocks formed by connecting a plurality of battery modules in series are connected in series. A module-voltage detection part 21 outputs voltage (measured values) V1' of the battery modules using a reference potential VSS as a reference on the basis of the voltage V1 of a prescribed battery module i and on the basis of a voltage V2 across one end of the battery module i and the reference voltage VSS of the prescribed end of the set battery. A correction expression (V1'=K1.V1+K2.V2+K3) in which K1, K2 and K3 are refer to constants and the constants K1 to K3 are stored. The expression is computed. The corrected module voltage V1 is calculated.