This backing member for a friction stir spot welding device is provided with a support surface that comes into contact with the back surface of an overlapping portion of a plurality of sheet members. The support surface includes: a base surface which holds the overlapping portion between a shoulder portion of a tool and the base surface; and a projected surface which is projected further toward the overlapping portion side than the base surface, and holds the overlapping portion between a pin part of the tool and the projected surface.
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