weaving a welding torch in a groove;

detecting an electric change amount at a time of the weaving;

calculating an average value of the electric change amount for every one interval, the one interval being a predetermined period;

extracting information on protruding change in the groove based on the average value; and

following the welding line based on the information on protruding change, wherein

the electric change amount includes, as a parameter, at least one of a welding current detection signal and an arc voltage detection signal,

an upper limit limiting value is calculated by adding a predetermined upper limit range value to a central value, and a lower limit limiting value is calculated by adding a predetermined lower limit range value to the central value which is the average value of the electric change amount in an immediately preceding interval of a target measurement period, and

when the average value in the target measurement period exceeds the upper limit limiting value or falls below the lower limit limiting value a predetermined process is performed.