PURPOSE: To reduce the weld defect by providing a measuring instrument of the number of spot weldings and providing a storage device of the voltage to fit the number of weldings and further, setting the voltage of the next spot welding based on a measured result of the number of weldings, etc.

CONSTITUTION: A voltage adjusting device main body 2 is connected with plural resistance welding machine 6 via a transformer 4 and a breaker 5 for the wiring and the instrument 7 to measure the number of spot weldings of each welding machine 6 is arranged. Further, a memory device 8 to decide the voltage of the next cycle time based on a value of the number of weldings counted with the measuring instrument 7 and its transition is provided and the proper voltage corresponding to the number of weldings is stored therein in advance. The timing when the welding is performed with each welder 6 overlapped is stored in advance and the proper voltage to solve a voltage drop at the time of welding can be supplied to a controller 9 by this mechanism. Accordingly, since the supply voltage is always properly maintained, the weld inferiority is reduced.