A one-sided spot welding device of the present invention includes a gun body portion including: an upper body disposed at an end of an arm of a robot and a hollow lower body disposed at a lower portion of the upper body; a power supply portion disposed within the lower body, a welding electrode portion i) disposed at a lower side of the lower body, ii) supplied welding power from the power supply portion, and iii) configured to weld welding base metals together by electrical resistance welding; a coated metal band supply portion supplying a coated metal band between the welding base metal and the welding electrode; and a ground electrode portion, electrically connected to the power supply portion and disposed at the other lower side of the lower body for contacting the welding base metal.
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