PURPOSE: To monitor a shield state easily without restraining welding operation, by finding arc length obtained from the operation of a welding voltage and a welding current, and comparing it with actually measured arc length and thus monitoring a shield state.

CONSTITUTION: A welding voltage between a body 1 to be welded and a chip 2 is detected by a voltage divider 12, and a welding current is detected by a shunt 13 to input those detected values to an operator 15. The operator 15 operates arc length L1 from the current and voltage. An arc length detector detects and inputs actual arc length L2 to the operator 15. The operator 15 operates an absolute valueL1-L2, which is compared with a value α set while an operation error of L1 and an actual measurement error of L2 are considered. WhenL1-L2<α, it is judged that the atmosphere wherein welding is carried out is close to that of pure Ar, and its signal is inputted to an alarm display device 20. In this case, the display device 20 generates no worning. WhenL1- L2>α, it is judged that the atmosphere contains impurities, and its signal is inputted to the alarm display device 20. In this case, if the impurities are water, L1<L2. The display device 20 generates an alarm.