A converter circuit includes three-phase unit converters connected in cascade in two stages for supplying electric power to a load. In a region in which the converter output voltage ratio is small, the firing phases of both converters are so set that a phase difference of 120.degree. makes appearance therebetween, while in a region in which the converter output voltage ratio is large, the firing phases are so set as to have a phase difference of 60.degree. therebetween, whereby an angular difference of 90.degree. to 30.degree. is caused to occur constantly between the phases of voltage ripple components of the unit converters. Power factor of a power supply source is improved while ripple in the output voltage of the converter circuit is reduced.