To reduce the number of fixed works to a floor form of void forms.

Truss reinforcing bars 10 connecting an upper main reinforcing bar 11 to a lower main reinforcing bar 12 with a diagonal reinforcing bar 13 are fixed on a slab floor form 3 at a required interval in parallel with the void form. A void unit 20 is set between the truss reinforcing bars 10, the void unit 20 is formed by arranging the void forms 1 on a reinforced cage 21 to fix, a hold-down frame 5 is assigned to the upper surface of the cage, both end locking bars 5a are locked with the truss reinforcing bars 10, and an uplift movement resulting from concrete casting is prevented. When the void forms 1 are fixed through the truss reinforcing bars 10 long arranged on the floor form 3, each truss reinforcing bar 10 fixes at least both ends thereof to the floor form to prevent the uplift movement resulting from the concrete casting, and when the void forms 1 are locked with the reinforcing bars 10, the void forms 1 are also fixed to the floor form 3. Then, when the void forms 1 are easily locked with the truss reinforcing bars 10, workability is promoted.