Abstract: A dc resistance welder includes a first rectifier circuit for converting the ac power from an ac power supply into dc power, an inverter for converting the dc power into ac power having a desired frequency, a welding transformer connected to the inverter, a second rectifier circuit for rectifying the output current of the welding transformer and for supplying welding current to a workpiece, a first element for setting welding current in accordance with the nature of the workpiece, a second element for sensing actual welding current for the purpose of sensing magnetic saturation of the welding transformer, and a third element for comparing the welding sent by the first element with the actual welding current sensed by the second element and for varying the output frequency of the inverter by inverting the direction of the output current of the inverter when the actual welding current reaches the set welding current.