PURPOSE: To simplify the setting of welding condition and to improve the working efficiency, by controlling the pulse width of each of the first and second pulses of the pulse arc welding method within a range from zero to the maximum set vale, and, if the output is insufficient when only the first pulse is used, by generating the second pulse also.

CONSTITUTION: First of all, a base electric current IB is determined against the minimum welding electric current value, and only a pulse group (a) is generated by equalizing the pulse frequency of the pulse electric current to the frequency of the AC power source. When the welding output is increased, the pulse width of group (a) is increased and the width is maintained at the maximum value (d) which is suitable to the diameter of wires and quality of materials, and then, the width of the group (b) is continuously increased from zero. Even in this case, the width of the group (b) is maintained at the maximum value (d). When the apparatus can use other pulse groups, it is necessary to repeat the same process and to increase the electric current IB after all pulse widths reach the value (d). When the value (d) is determined, the pulse frequency is also determined in accordance with the welding output. Therefore, when the pulse electric current IP is to be reduced or increased when welding Al alloys or iron metals, it can be made by setting the value (d) small or large.