Reinforcement body for a cast concrete floor slab provided with stiffening ribs, having a foundation reinforcement (1), a distribution reinforcement (2) and at least one displacement body (8) e.g. of polystyrene foam, arranged between these reinforcements which delimits cavities to accept the stiffening ribs. The displacement body is rigidly fixed between the reinforcements by means of spacer wires (7) connecting the foundation reinforcement to the distribution reinforcement (2) at a selectable distance, e.g. by passing through the displacement body, so that the foundation reinforcement, the distribution reinforcement and the displacement body form a dimensionally stable unit. Additional reinforcement elements (10, 11, 15), can be placed in the rib cavities (9). The cavities are either formed in a one-part displacement body or formed between several displacement bodies and having predetermined directional course in the reinforcement body and predetermined width, in accordance with the static requirements of the floor slab including the stiffening ribs.