In the step-up mode, mainly, two modes, that is, a charge mode and a discharge mode, are changed over to each other by the switching elements S1 to S4. In the charge mode, electric charge is stored in the reactor. In the discharge mode, stored electric charge is discharged to a load. In the parallel step-up mode, the charge mode and the discharge mode are executed in each of the first step-up/step-down circuit CNV1 and the second step-up/step-down circuit CNV2 independently of each other. That is, in the step-up mode, the two types of operation modes are changed by the two types of circuits, so four types of operation states are considered in total. Hereinafter, these operation states will be described with reference to FIG. 3 to FIG. 6.

FIG. 3 shows a view at the time when both the first and second step-up/step-down circuits CNV1, CNV2 are in the charge mode. In view of the operation of the existing step-up/step-down converter shown to the top left of FIG. 2, in the charge mode, the upper arm off and the lower arm on are set, so, in the electric power converter 11 shown in FIG. 3, 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 on state. 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 off state.