A joining device, for joining a shelf to a wall of a piece of furniture, the joining device comprising a cylindrical body that can be associated in a first hole adapted to be provided transversely in said shelf and provided with a first axial seat for the rotatable coupling of the first stem of a screw provided with a first crown-shaped head that can be arranged rotatably within a second seat adjacent to said first seat, wherein said cylindrical body is provided with a third seat, perpendicular and connected to said second seat, for the temporary coupling of a pinion, which interacts with said first crown-shaped head, which protrudes externally to said cylindrical body and is accommodated within a second hole adapted to be provided in said shelf