PURPOSE: To improve bead quality and to speed up welding by providing a change rate control circuit of a welding power source output current and arranging a current value decision circuit and changing an amplification degree of the current change rate control circuit based on its decision output.

CONSTITUTION: A high-frequency inverter circuit 4 to control output to a welding power source is provided and a secondary winding 7a of a DC reactor 7 is made to a current change rate detection means. The generation voltage of the secondary winding 7a is inputted to the current change rate control circuit 17 and added to a reference input signal of an output voltage setter 15 and subjected to negative feedback to the inverter circuit 4. At the time of performing short circuiting arc welding by using said welding power source, the current value decision circuit 18 changes the amplification degree of the current change rate control circuit 17 and a current waveform is controlled to prevent an incomplete short circuit from being generated. By this method, since the number of times of short circuits increase, the bead quality is improved. Since welding is stabilized, welding is speeded up accordingly.