PURPOSE: To improve the welding quality and welding efficiency, by measuring the time from start of power supply till the change in resistance between electrodes becomes the maximum, and adjusting the current standard so that the measured time may be equal to the set value.
CONSTITUTION: A current reference value 134 is outputted from a current setting part 33 and a current standard generating part 34, and the root mean square value determined through current transformer 36, current detecting part 37, and effective value calculating part 38 is compared with the reference value in a comparator 39, and the set current is designed to flow into members 13a, 13b to be welded by way of phase control part 40 and thyristor 31. The weld time indicating the maximum resistance is measured through maximum voltage and current detecting parts 44, 45, resistance calculating part 46, and resistance change detecting part 47, and is judged by a counter 48. The average of n times is taken by memory part 49 and reference value calculating part 50, and the reference value is calculated, and set in a reference value setting part 51. The counting 148 by the counter 48 and the reference value 151 are compared with each other in the comparator 52, and the deviation is inputted in a current correction rate detecting part 53, and the reference value 134 is automatically corrected by the current correction rate 153, so that the welding current may be corrected.