One-side resistance spot welding method and apparatus for carrying out the same

A one-side resistance spot welding method includes a preheating step followed by a welding step. In the preheating step, an electric current having a smaller current value than a welding current is fed from a first electrode to second electrodes such that a substantially annular high-temperature region coaxial with the first electrode and having a higher temperature than neighboring parts is created in a workpiece due to resistance heat generation at a position located near a tip end of the first electrode. In the welding step, the annular high-temperature region forms a shielded current path due to its high electric resistance and serves to guide a welding current from the first electrode to flow into a bottom steel sheet of the workpiece via the shielded current path, thereby allowing the welding current to subsequently advance along the bottom steel sheet and reach the second electrodes via a top steel sheet.