To provide a device for preventing the fall of a ladder, which can secure safety of a usage state of the ladder to the utmost by ensuring the state of attachment to the ladder and enhancing convenience of handling in transportation, without the need for the application of special working to the existing ladder.

In the ladder W, a large number of rungs W2 are laid at predetermined vertical intervals between right and left supports W1 facing each other at a predetermined interval. The device for preventing the fall of the ladder includes: a pair of right and left supporting rods 2 which can be attached to/detached from the right and left supports W1 of the ladder W and which are elongated out to the lateral outward direction of the support W1; a pair of right and left fixing means 4 which are assembled to the pair of right and left supporting rods 2 and fixed to the supports W1 of the ladder W, respectively; and right and left outrigger means 6 which have bases 5 provided in a suspended state in portions, elongated out in the lateral outward direction of the support W1, of the pair of right and left supporting rods 2 and brought into contact with the ground together with a lower end of the ladder W, and which can be positionally adjusted in a vertical direction.