The first step-up/step-down circuit CNV1 is established between the first battery B1 and the high-voltage line 26 (output line). The second step-up/step-down circuit CNV2 is established between the second battery B2 and the high-voltage line 26. 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.

Based on the above-described definition of the upper arm and the lower arm, the arm (S1 arm) including the switching element S1 and the diode D1 is an upper arm when viewed from the first and second step-up/step-down circuits CNV1, CNV2. The arm (S2 arm) including the switching element S2 and the diode D2 is an upper arm when viewed from the first step-up/step-down circuit CNV1, and is a lower arm when viewed from the second step-up/step-down circuit CNV2. In addition, the arm (S3 arm) including the switching element S3 and the diode D3 is a lower arm when viewed from the first and second step-up/step-down circuits CNV1, CNV2.