PURPOSE: To enable the easy positioning of a tightener even if a main body and the tightener are not in proper positional relation to each other, by floatingly supporting the tightener and using a guide member to position the main body, the tightener and an element to be tightened on the main body.

CONSTITUTION: A device for supporting the nut tightener N of a robot in an automobile assembly line comprises a flat base plate secured on the multi-joint arm of the robot, a movable plate 12 supported in a floatable manner by the base plate, and a locking member for securing the movable plate in an optional position. A mechanism 14 for setting the floated quantity of the movable plate 12, an engaging member 16 capable of being engaged in the engaging hole 15 of a strut P to be assembled with an automobile body B, and a guide member 17 to self-centripetally engage in the positioning hole 3 of the strut tower 2 of the automobile body are provided. The locking member 13 is placed between the base plate and the movable plate 12.