Abstract: In spot welding, the condition tolerance for applying a large current in a short time into an aluminum alloy plate or the like is narrow, and also nugget crack sometimes occurs in spot welding. In the case of a mild steel plate, the expulsion sometimes occurs, and thereby a poor weld portion is formed. Due to a pickup removal operation from an electrode tip, the operating ratio is reduced. Here, a PAM control method is employed for raising or dropping a rectified and smoothed voltage EDC output to an inverter circuit, according to a certain set-up ratio in a predetermined time, instead of a conventional PWM control method. By this control method, the nugget can be formed in condition of low heat dissipation and high thermal efficiency. In addition, energy saving and power saving are achieved by using the PAM control method.