To provide easy attachment and prevent local load from being applied to the mounted part of a back door.

This ladder device consists of a ladder body 1 having a plurality of steps, an upper holder 2 which is provided on the upper part of the ladder body 1 and is mounted so as to be capable of relative rotation to the ladder body 1 in the three-dimensional direction, a lower holder 3 which is provided under the ladder body 1 and is mounted so as to be capable of relative rotation within a plane forming the ladder body 1, a puller mechanism 5 which is provided under the ladder body 1 and operates so as to draw the lower holder 3 in the longitudinal direction of the ladder body 1, and a locking device 6 which is provided at the upper holder 2 and regulates the oscillating movement of the upper holder 2.