To prevent uplift of a void form and facilitate construction.

In a deck plate 20, truss bars 3 are welded to a base plate 2 of a thin steel plate at an appropriate interval, and upper end bars 4 and lower end bars 5 are welded to the truss bars 3. A columnar void form 1 made of foamed resin having an elliptical cross section and having grooves in both side faces is pushed into the truss bars 3 and fixed. The truss bars 3 of the deck plate to be connected are fitted to the grooves 11 of the void form 1, and are pushed utilizing the elasticity of the foamed resin, and a joint 21 of two deck plates is engaged to be connected and fixed. An assembled body obtained by repeating this process to have a needed size is carried into a building by a crane etc. and installed. Then, concrete is placed to construct the void slab.