Abstract: A device for securing the shafts of buildings, for example light shafts, against forcible intrusion or passage. The device includes at least one hollow blocking bar which spans the shaft transversely and the two ends of which are held in respective anchor members attached to the wall of the shaft with covered bolts and which contains a hollow security rod of substantially the same length. The blocking bar has the form of a polygonal prism the ends of which are received in form-fitting manner in respective recesses of associated respective attachment bases in such a way that they lie in opposing alignment to the mounting bolts of the base and are secured against pull-out by a latch member lock.