PURPOSE: To reduce the fraction defective in welding by monitoring the max. electric resistance over the main range of weld time and controlling an welding work.

CONSTITUTION: The max. electric resistance of the heat affected zone 16 of after contact resistance range is detected and the upper limit value for preventing an overheat and the lower limit value preventing a welding heat shortage are stipulated as the allowable range of the max. electric resistance in advance. In case of the detected max. electric resistance exceeding the upper limit value an alarm sound is given or the place deemed to be a defecting welding is clarified by coloring it by a color spray, or the welding machine is stopped or the welding is controlled so that the max. electric resistance comes in the allowable range. At the time when the max. electric resistance is below the lower limit value it is short of welding heat, so the control that an alarm is given, the current is increased or the voltage is increased or welding time is extended, etc. is executed. The fraction defective in welding can thus be reduced.