To enhance accuracy of equalization while taking advantage of the feature of equalization time shortening by a transformer coupling system, in the equalization control of the voltage of a battery pack constituted by connecting a plurality of battery cells.

A transformer balance circuit 105 performs individual equalization operation of each stack 103, via a switching element 110, a transformer 109, and a rectifier circuit 111 connected individually. A DSP107 calculates the voltage of the stack 103 based on the voltage of each battery cell 102 detected by a voltage monitoring section 106, determines a stack 103 for which equalization operation must be performed based on the voltage thus calculated, and makes a switching element 110 corresponding to the stack thus determined perform the equalization operation by instructing a switching operation corresponding to the equalization operation.