Abstract: A guard rail comprising stiffening posts, a bottom rail and a top rail connecting the posts, is fastened to a building element, for instance a concrete slab, by a fastening device. The device comprises an anchor plate permanently secured to the building element and having an upper spherical support surface. A base plate has a lower support surface received by the support surface of the anchor plate. The upper portion of the base plate has a projecting collar which receives one of said posts in vertical position. The amount of horizontal offset of the collar is such that the posts and the bottom rail may be in contact with the building element.