A friction stir spot welding apparatus according to the present invention includes: a rotary tool (10A); and a perpendicular-to-plane detector (20A), which detects whether or not the rotary tool (10A) is positioned to be in a perpendicular-to-plane state relative to a joining target portion of workpieces (50). The perpendicular-to-plane detector (20A) measures distances to at least three measurement positions set around the joining target portion by means of, for example, position sensors (21). The measurement positions are set to be positioned at the vertices of a polygon on a reference plane to which forward and backward movement directions of the rotary tool (10A) are normal. The perpendicular-to-plane detector (20A) detects that the rotary tool (10A) is positioned to be in the perpendicular-to-plane state relative to the joining target portion of the workpieces (50) if all the distances to the measurement positions are equal to each other.
