Abstract: A support device for an instrument on a loading/offloading tower of a tank of a ship is disclosed. The tower has multiple vertical masts linked to one another, pairwise, by struts extending on a strut axis, and a distal portion extending on a first axis, the distal portion being terminated by a first surface capable of forming a first surface contact with a strut extending on its strut axis substantially at right angles to the first axis. The installed first surface has a degree of freedom in rotation about the strut axis, a proximal portion extending on a second axis substantially at right angles to the first axis, linked to the distal portion. An installed fixing surface immobilized by fixing to the instrument has a degree of freedom in rotation about the second axis and a degree of freedom in translation on a third axis parallel to the second axis.