PURPOSE: To enable the easy lifting and lowering of a ladder body by connecting the ladder body to one end of a horizontally supported step at the side of a hull in such a way as to be capable of turning vertically, and providing a positioning cylinder energized toward an expansion side via a compressed gas between the step and the ladder body.

CONSTITUTION: A step 3 and a ladder body 4 are stored upright at the side of a motor boat 1 equipped with an outboard engine 2. Also, the step 3 is so coupled to a hull 5 as to be capable of free turning along the long side thereof via a bracket 6, and kept capable of free turning between up-right and horizon tal positions with a positioning cylinder 7. In addition, the ladder body 4 is connected to the aft end of the step 3 via a hinge 8 and so made as to be capable of turning and being supported at a desired angle with a positioning cylinder 9. Each of the positioning cylinders 7 and 9 is so constituted that oil chambers A and B at both sides of a piston 12 are continuous to an oil accumu lator 15 sealing compressed gas via respective valves 17 and 18, and each of the valves 17 and 18 is operated with an operation lever 23 for expanding and contracting the ladder body 4.