To suppress an unnecessary short-circuit avoiding operation resulted from the axial shaking of an electrode under machining by providing a short- circuit back control means for performing the short-circuit avoiding operation when a judgment result that the short-circuit duration exceeds a short-circuit judgment standard time is obtained.

A short-circuit judgment standard value changing means 11 changes the short-circuit judgment standard time stored in a variable memory 10 according to the rotating speed of an electrode 1. When a short-circuit is detected by a short-circuit detecting circuit 7, the duration of the short-circuit or the short-circuit duration is measured by a short-circuit duration judging instrument 9. Further, the short-circuit duration is compared with the short- circuit judgment standard time stored in the variable memory 10. When the short-circuit duration exceeds the short-circuit judgment standard time by the comparison, this is transmitted from the short-circuit duration judging instrument 9 to a short-circuit back controller 8. Then, the short-circuit avoiding operation of a rotating device 4 is controlled by the short-circuit back controller 8.