Abstract: The invention relates to a prefabricated concrete component system for forming ground-bearing stairways, platforms, terraces and analogous structures of one or more layers. It is the object of the invention to be able to construct, from a few basic components, a durable and safe ground-bearing structure of the desired shape. Such a component system comprises at least three different basic components (1, 2, and 3), which are used together or separately and which are slab components provided with upright supports. The basic components of the component system according to the invention are so designed that one or two adjacent slab components in the same constructional layer form at least two support and fastening surfaces for the next higher slab component. Thus a sturdy and safe structure is obtained. The shape of the components enables them to be cast in a single-part casting mold, from which the component will detach owing to vertical surfaces.