PURPOSE: To compensate variation of drop time by holding a plunge time of a stud against a workpiece substantially at an ideal constant value in stud welding.

CONSTITUTION: In each process of welding a stud 16, an average of plunge time actually generated in the prescribed time of welding previously practiced is calculated every practiced time. Upon a variation of average of plange time, a point of time of command to drop a solenoide 12 is made advanced or retarded to adjust a drop time, and thus aging, abrasion mechanical change and others in the welding machine 20 are compensated. Thereby, the plunge time is automatically held very close to an ideal value.