Current components respectively flowing through the switching elements S1 to S4 in the drawing are shown to the top left of FIG. 3. The switching element S1 is in the off state. The current I2 from the second battery B2 flows through the switching element S2. The current I1 from the first battery B1 and the current I2 from the second battery B2 flow through the switching element S3. The current I1 from the first battery B1 flows through the switching element S4.

FIG. 4 shows a view at the time when both the first and second step-up/step-down circuits CNV1, CNV2 are in the discharge mode. In the discharge mode, the upper arm on and the lower arm off are set, so the switching element S1 of the S1 arm that is the upper arm of both the first and second step-up/step-down circuits CNV1, CNV2 is in the on state. The switching elements S2 to S4 of the S2 to S4 arms that are the lower arms of at least one of the first step-up/step-down circuit CNV1 or the second step-up/step-down circuit CNV2 are in the off state.

At this time, when one of the S3 arm and the S4 arm that are the lower arms for the first step-up/step-down circuit CNV1 is set to the off state, the loop circuit that connects the first battery B1 with the first reactor L1 is interrupted, so the mode is changed to the discharge mode. Similarly, when one of the S2 arm and the S3 arm that are the lower arms for the second step-up/step-down circuit CNV2 is set to the off state, the mode is changed to the discharge mode.