To save the labor for storage or laying of adders in a hold by supporting a second unit ladder to the tip of a support arm mounted on the inner wall of a hold in such a manner as to be rotatable so that the lower end of a first unit ladder and the upper end of the second unit ladder are pinned to both ends of a connecting member, respectively.

A connecting member 11 has a vertical elliptic hole 12, and a pin 10b is connected so as to be fitted to the elliptic hole. Namely, although the connecting member 11 is rotated around a pin 10a when a second unit ladder 6b is stored, the pin 10b situated in the lower part is moved along a virtual line (b) around a virtual point θ. Since the pin 10b is slid within the elliptic hole 12 at that time, the second unit ladder 6b can be smoothly stored. The elliptic hole 12 and the pin 10b may be formed on the upper end of the second unit ladder 6b and on the connecting member 11 side, as occasion demands, respectively. Further, the pin 10a for connecting the upper end of the connecting member 11 to the lower end of the first unit ladder 6a may be formed into the same structure.