The first step-up/step-down circuit CNV1 includes the first battery B1, the first reactor L1 and the first capacitor C1. The second step-up/step-down circuit CNV2 is provided in parallel with the first step-up/step-down circuit CNV1, and includes the second battery B2, the second reactor L2 and the second capacitor C2. The switching elements S1 to S4 turn on or off to switch between storage and discharge of electric charge of the first and second reactors L1, L2.

In this way, the first step-up/step-down circuit CNV1 and the second step-up/step-down circuit CNV2 have such a circuit configuration that the switching elements S1 to S4 and the diodes D1 to D4 are shared. However, the functions of the elements are partially different between the first step-up/step-down circuit CNV1 and the second step-up/step-down circuit CNV2. Specifically, the functions of the switching elements S1 to S4 and diodes D1 to D4 as “arms” are different, between the first step-up/step-down circuit CNV1 and the second step-up/step-down circuit CNV2. The arm indicates a combination of each switching element and a corresponding one of the diodes connected in antiparallel with the switching element.

FIG. 2B shows the circuit diagram of an existing step-up/step-down converter. In this converter, two arms are divided into an “upper arm” and a “l(fā)ower arm”. Conventionally, the lower arm indicates an arm that establishes a loop circuit including a battery and a reactor. The upper arm indicates an arm that establishes a circuit that connects both a battery and a reactor to a load.