A friction stir spot welding device which welds an object to be welded, by softening the object by friction heat and by stirring the object, has a tool with a cylindrical shape, the tool being rotatable around an axis thereof, advanceable along a direction of the axis, and retractable along the direction of the axis, a temperature detector which detects a temperature of the tool when a welding step for the object starts, and a controller configured to compare the temperature of the tool which is detected by the temperature detector to a predetermined reference temperature to set a plunging force applied by the tool to press the object.