The friction stir spot welding apparatus according to the present invention is a surface for detecting whether or not the position of the rotary tool (10A) and the rotary tool (10A) is in a state of being perpendicular to the joining portion of the workpiece (50). And a direct detection unit (20A). The surface straightness detection unit (20A) measures the distances to at least three measurement positions set in the vicinity of the joint site by using, for example, the position sensor (21). These measurement positions are set so as to be positioned at the vertices of the polygon on the reference plane whose normal direction is the advancing / retreating direction of the rotary tool (10A). The face straightness detection unit (20A) detects that the face straightness is present when the distances to the measurement positions are all equal.
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