PURPOSE: To detect that a surface defect is scarfed away by setting the electric current over the output current for generating a pilot arc and below the output current for generating a main arc and outputting a command signal for scarfing when the current of a plasma torch exceeds the set value.

CONSTITUTION: The electric current over the output current for generating a pilot arc and below the output current for generating a main arc used for scarfing is beforehand set in the stage of scarfing a surface flaw with the plasma arc in accordance with the command signal for scarfing outputted from an automatic scarfing device. The current to be supplied to the plasma torch is detected and when the current above the set value is outputted, the signal indicating that the scarfing by the plasma arc is accomplished is outputted. Whether the scarfing is accomplished or not is judged by combining said signal and the command signal for scarfing from the scarfing device.