We developed a frequency-controlled inverter power supply for spot welding machines. The welding currents reached 10 kA or higher, while impedance at the weld was extremely small, at 1 mΩ or less. These conditions demanded welding power supplies capable of providing low voltage at high current. To develop such a power supply, we investigated how to control the welding current using an inverter power supply by installing an internal transformer with a large winding ratio. However, due to issues concerning the global environment and energy conservation, inverter power supplies require increasingly higher efficiency. One method of improving efficiency is to match the impedance between the inverter power supply and the load. The leakage impedance of the inverter transformer, however, is dependent on frequency and current. In addition, the extremely low load (1 mΩ or less) makes such impedance matching difficult to implement. This paper presents the results of an investigation into high-efficiency power control between an inverter power supply with an internal transformer (with a large winding ratio) and a load of no more than 1 mΩ. The results confirmed that inverter output power that maximized frequency and matching were possible.