In the present invention, when spot welding a cold rolled steel sheet and an aluminum alloy sheet, pre-electrification is performed ahead of time in conditions wherein the sheet thickness (ta) of the aluminum alloy sheet and the sheet thickness (ts) of the cold rolled steel sheet that are to be joined are in specific ranges, dust is not generated between the steel sheet and an electrode, and the aluminum alloy sheet side is not caused to melt, and afterwards, main spot welding is performed. The welding conditions of the main spot welding and the pre-electrification further satisfy a relational expression that is determined by each electrification condition and each sheet thickness.