To reduce cost by regularizing a void form unit P.

This void form unit P is constructed by arranging a plurality of void forms 4 in parallel, and inserting to integrate the void forms 4 arranged in parallel, into a reinforcing cage A with longitudinal and lateral reinforcements 1, 2, 3 disposed. The void form unit P is placed through legs 1c on a floor form 7 with lower end main reinforcements 8 arranged, and holes 7a are formed in the floor form 7. Hook members 5 are inserted through the holes 7a and locked to the floor form 7. The hook members 5 and the reinforcements 2, 3 of the void form unit P are then connected by iron wires 6 to fix the void form unit P to the floor form 7. If the legs 1c are integrated with the reinforcements, site work for height adjustment of the void forms is dispensed with by previously providing the legs 1c in a factory or the like. Workability is thereby improved, and the spacing of the void forms and floor form is fixed. At this time, the hook members 5 are positioned between the void forms 7 arranged in parallel.