To reduce volume during conveyance.

This void form 11 arranged inside a slab for forming a hollow inside the slab is formed of a cylindrical tube symmetric to the pipe axial face, and the cylindrical tube is divided into two sections 11a and 11a from the symmetry plane. One 12a of the divided ends is shaped into a U-shape, while the other 12b is shaped flat. The flat end 12b is fitted to the U-shaped end 12a to be connected together. In this process, the connection end parts are crimped to be firmly connected together. Because a plurality of half divided cylinders 11a can be overlapped mutually as shown in a figure (a), the void forms 11 can be conveyed from a plant in the overlapped condition, and in a work field, the connection parts 12a and 12b are connected together for forming the void form 11 as shown by the figure (b), and then, the void form 11 is arranged on the slab form.