PURPOSE: To perform automatically the judgement of whether the measurement of current and voltage and the values are within proper range or not by comparing the upper limit value and lower limit value of which the arithmetic mean value of the measured value of welding current and voltage is set in advance and by alarming the abnormality in case of exceeding the range.

CONSTITUTION: The current and voltage of arc welding circuit are measured by each measuring circuit 5, 6 via each input circuit 3, 4 on each preset sampling period with the input of the signal transmitted from a measuring conditions setting circuit 7 at welding work time. The arithmetic mean value of the respective measured value is calculated by arithmetic circuits 8, 1 and discriminated with comparing by means preset upper and lower limit values and comparison discriminating circuits 10, 11. In case of the mean value being within the range of the upper and lower limit values, the welding state is taken as good and the output signal of the abnormality is not inputted in an abnormality alarming device 14 or the display of normality is held. On the other hand, the abnormal signal is inputted in the abnormality alarming device 14 in case of the mean value exceeding the upper and lower limit values and it is displayed by a lamp buzzer, etc. or recorded by a printer, etc.