For connected-in-parallel N DC-DC converters (1 to N, where N is 3 or more), a first converter group on which PWM switching is performed with a first carrier, and a second converter group on which PWM switching is performed with a second carrier having a phase different from that of the first carrier, are provided. One or more of the DC-DC converters (1 to N) are assigned to each converter group. A plurality of drive modes in which the number of the DC-DC converters that are driven and the converter groups are set, are provided. The drive modes are switched through comparison between a preset threshold value and a total output of the N DC-DC converters (1 to N), and a ratio between total outputs in the first and second converter groups is set to fall within a predetermined range.