PURPOSE: To ensure the stability of arc from low current to high current and to improve the efficiency of welding work, by changing a peak current and a pulse frequency according to average welding current.

CONSTITUTION: A peak current generating circuit 23 calculates in a peak current setting circuit 24 basing on the value of average output current set by a welding current setting circuit 25, and compares a selected peak current with a peak current measured by a current measuring device 18 and extracted by a peak current detecting circuit 22, and generates a peak signal to make the two coincide with each other. A base signal generating circuit 27 compares a base voltage set by a base voltage setting circuit 26 and an actual base voltage of a base voltage generating circuit 28, and generates a base signal to make the two coincide with each other. A pulse frequency setting circuit 20 calculates and selects the pulse frequency basing on the value of average welding current set by a welding current setting circuit 25. It further calculates base time from a peak signal generating circuit 23, and controls a peak base changeover switch 21.