PURPOSE: To facilitate riding onto a boat from the under-water by connecting a ladder main boat to the boat so that it may run only around a rotational axis parallel to its side, and maintaining the body vertical to the boat even if the boat is inclined when a user is on the body.

CONSTITUTION: A ladder main body A has a plurality of lateral rods extending laterally and connected thereto between a pair of right and left columns 1, 2 extending in the vertical direction. The rod 3 is bent in a recessed form such that the central part is recessed rather than the opposed ends. The columns 1, 2 and the rod 3 are made of aluminum pipes and are given with float by closing the opposed ends. A mounting member 6 is connected to the upper end of one 1 of the columns via a hinge 7, while the connection pin 8 arranged at the lower face of the member 6 is inserted into a through hole 5 for insertion of an oar support pin bored at a frame member 45 of a boat B. It is thus possible to maintain the body vertically even if the boat B is inclined.