PURPOSE: To detect the resistance drop between objects to be welded with good accuracy by correcting the average value of the inter-electrode voltage value at each of power source cycles in the stage of short circuiting and applying current with a constant and determining the difference between said value and the voltage value during welding as a detected value of the voltage between the welding electrodes.

CONSTITUTION: The voltage between welding electrodes for resistance welding at each of power source cycles when the electrodes are short circuited and are applied with current is detected and is inputted with an AC integration circuit I , a sample holding circuit II and an A/D conversion circuit II to a microcomputer IV. On the other hand, the welding current is inputted to a current transformer V and a zero point detection circuit IV which detects the point of the time when the welding current begins to flow and the point of the time when the flow ends. The detected points of the time are inputted to the microcomputer IV. The number of welding spots is inputted to the microcomputer IV. The number of welding spots is inputted to a waveform shaping circuit VIII, which communicates the microcomputer IV with the spotting condition to correct the detected voltage in th stage of short circuiting. The differential value between the detected values during the actual welding and in the stage of short circuiting is obtained with the microcomputer IV and is determined as the detected value of the voltage between the welding electrodes.