To provide a firm connection, when cast-in-place concrete is placed onto concrete panels each with a hollow hole to construct a floor panel, by allowing part of the cast-in-place concrete to be uniformly placed into each hollow hole by a desired amount, and keeping the density of the concrete uniform and high.

A closure plate 10 of sufficient size to close the hollow hole 1 in the hollow concrete panel P is provided which is inserted into the hollow hole 1. Three to five leg pieces 12 extending from the edge of the hole in a direction perpendicular to the closure plate 10 are provided. The end of each leg piece 12 is formed with a hook 13 bent outwardly at right angles and capable of hooking the end face of the hollow concrete panel P.