As described above, the CNV controller 13 controls the on/off operations of the switching elements S1 to S4 through the PWM signals PWM1, PWM2. Generation of the PWM signals PWM1, PWM2 will be described. The CNV controller 13 executes feedback control for obtaining a difference between the measured output voltage VH of the electric power converter 11 and a target value VH* of the output voltage and reducing the difference. If a feedback gain is excessive in the control, hunting that an intended parameter (output voltage VH) oscillates occurs, so the output voltage VH is gradually brought close to the target value VH* over multiple periods.

For example, when the controller 22 determines a request to increase output torque on the basis of, for example, an increase in pedal stroke amount received from an accelerator pedal sensor, the controller 22 raises the target value VH* of the output voltage of the electric power converter 11. The CNV controller 13 sets target command values for the first step-up/step-down circuit CNV1 and the second step-up/step-down circuit CNV2 on the basis of the raised target value VH* as indicated by the broken line in FIG. 7. FIG. 7 shows only the process of generating the PWM signal PWM1 for the first step-up/step-down circuit CNV1.

The CNV controller 13 sets the rate of increase and gradually brings the command wave to the target value by raising the command wave of the PWM signal once every period. The rate of increase may be a predetermined upper limit value of the feedback gain. As a result of a rise in the command value over multiple periods, an on-duty period (hereinafter, simply referred to as on period) of the PWM signal PWM1 is gradually reduced.