PURPOSE:To provide a resistance welding electric source, in which control mode being suitable to the quality of a material to be welded can be selected. CONSTITUTION:This electric source is provided with a CPU part 22D inputting and storing voltage, current and power as the welding condition, as setting values for feedback control to execute arithmetic control, a current detector 20 connected with the output side of a power transistor 4 to detect the current, a welding electrode 5 connected with the output side through the current detector, an electrode voltage detecting part and an amplified multifying part 21 amplifying the voltage from the voltage detecting part and the current from the current detector 20, respectively and also calculating the power from the voltage and the current. Then, this electric source is provided with a voltage, current and power control part 22 inputting the voltage, current and power for feedback from the amplified multifying part 21 and controlling while comparing each feedback value and the setting value for feedback control and executing the feedback to the transistor to control either one of three control modes and a welding condition setting part 23 for giving the voltage, current and power control part the selected output of the control mode. The feedback control is executed to the transistor with the selected control mode.