Conventional floor panel made of concrete is produced by pouring concrete into a rectangular formwork to be in a form of plate having a small thickness.  During the pouring of the concrete into the formwork, some reinforcing materials such as expand metal and reinforcing bar have been previously placed in the formwork to be integrated within the resulting hardened concrete in order to increase rigidity of the resulting panel. 
The expand metal is placed in approximately whole of the plane of the floor panel.  The reinforcing bars are placed, for example, along outer periphery of the floor panel, or on the center of the floor panel wherein the reinforcing bars are crossing to each other (see, for example, Patent Literature 1). 
Moreover, as other embodiment of floor panel, a floor panel made of steel, which contains lightweight concrete therein, is known, which comprises a top plate made of steel and a bottom plate made of steel, which are placed facing to each other, to form a hollow structure, and joined together at their peripheries, wherein lightweight concrete is poured and then hardened in the hollow structure (see, for example, Patent Literature 2). 
Each of the floor panels described above is used as an indoor flooring material.  Therefore, a smaller thickness is required.  Herein, concrete itself has insufficient rigidity, therefore, it causes problems.  For example, reinforcing material made of metal is embedded in the concrete, in order to reinforce the concrete, and the steel plates cover whole of the lightweight concrete.