To provide an auxiliary device capable of preventing shaking of a long ladder such as an expanded ladder and increasing the strength of a ladder main body.

First, upper and lower ladders are fixed in a step part. Secondly, a metallic bar is hooked from above and below at the rear side of the ladder, and the device for pushing out its central part to the outside of the ladder and fixing it is attached so that the ladder main body stretches. Since it becomes a bow shape in the reverse direction to essential bending, swing is suppressed and the strength is increased to prevent the ladder from shaking due to these comprehensive effects.