Abstract: Disclosed is an invertor type DC welding machine incorporating constant-voltage control to assure a high and reliable quality of weld. The pulse width of the high-frequency output of the invertor circuit is controlled at the invertor switching rate by the constant-voltage control on a feedback loop basis in which a DC weld voltage applied to the workpieces to be welded between electrodes is detected and is compared with the value of a reference weld voltage to produce an error signal for controlling the operation of the invertor circuit, so that the DC weld voltage is maintained constant regardless of variations in the resistance of the portion to be welded during a welding operation. A timing system is further provided which momentarily monitors the change in the resistance between electrodes to terminate the welding operation at an optimum time after a nugget is produced in the porton to be welded.