Mobile anchoring and fall prevention device to making it possible to ensure the security of workers of any trade who operate from the inside of a building, on a wall, through a bay in said wall, while in the vicinity of empty spaces and without damaging the inner or outer existing structures (floor, wall, ceiling), comprising: - a rigid structure (7, 10, 12) to be placed opposite the bay, comprising a single post (7, 10, 12) intended to rest against the floor of the room in which the work is taking place and against the ceiling of this same room, said post being equipped with means (8, 9, 13, 14, 32, 4) for adjusting its length so as to apply its ends tightly against this floor and this ceiling, respectively; - a hollow member secured to said single post (7, 10, 12); characterized in that the device also comprises: - two arms hinged to the hollow member, intended to bear on the wall, above the bay and on either side thereof, and - an anchoring point secured to said hollow member, located between the two arms; - for forming said single post (7, 10, 12), the apparatus comprises: - a first telescopic bar, comprising a bearing pad for bearing against the ground; - a second telescopic bar to be engaged within the first telescopic bar, and on top of which said hollow member is fixed; - a third telescopic bar, or upper extension, intended to be engaged inside said hollow member and movable relative thereto, said third telescopic bar comprising a bearing pad intended to bear against the ceiling; said means for adjusting the length of the single post comprises: - said first, second and third telescopic bars (7, 10, 12), adapted to be fixed in a specific position relative to each other by a system of holes (8, 14, 32) provided in these telescopic bars (7, 10, 12) and by at least one pin (9, 13), said predetermined position being such that, in this position, said single post has a length slightly less than distance between the floor and the ceiling of the room in which the work takes place; - means for making fine adjustments to the length of said single post, with the possibility of tightening, acting at the level of at least one end of this post (7, 10, 12), said fine adjustment means making it possible to achieve tight application of said pads (1, 34) of the post against the floor and the ceiling, respectively. - a rigid structure (7, 10, 12) to be placed opposite the bay, comprising a single post (7, 10, 12) intended to rest against the floor of the room in which the work is taking place and against the ceiling of this same room, said post being equipped with means (8, 9, 13, 14, 32, 4) for adjusting its length so as to apply its ends tightly against this floor and this ceiling, respectively; - a hollow member secured to said single post (7, 10, 12); - two arms hinged to the hollow member, intended to bear on the wall, above the bay and on either side thereof, and - an anchoring point secured to said hollow member, located between the two arms; - for forming said single post (7, 10, 12), the apparatus comprises: - a first telescopic bar, comprising a bearing pad for bearing against the ground; - a second telescopic bar to be engaged within the first telescopic bar, and on top of which said hollow member is fixed; - a third telescopic bar, or upper extension, intended to be engaged inside said hollow member and movable relative thereto, said third telescopic bar comprising a bearing pad intended to bear against the ceiling; - a first telescopic bar, comprising a bearing pad for bearing against the ground; - a second telescopic bar to be engaged within the first telescopic bar, and on top of which said hollow member is fixed; - a third telescopic bar, or upper extension, intended to be engaged inside said hollow member and movable relative thereto, said third telescopic bar comprising a bearing pad intended to bear against the ceiling; - said first, second and third telescopic bars (7, 10, 12), adapted to be fixed in a specific position relative to each other by a system of holes (8, 14, 32) provided in these telescopic bars (7, 10, 12) and by at least one pin (9, 13), said predetermined position being such that, in this position, said single post has a length slightly less than distance between the floor and the ceiling of the room in which the work takes place; - means for making fine adjustments to the length of said single post, with the possibility of tightening, acting at the level of at least one end of this post (7, 10, 12), said fine adjustment means making it possible to achieve tight application of said pads (1, 34) of the post against the floor and the ceiling, respectively.