PURPOSE: To prevent dislocations and unsteadiness from occurring to a double floor and to materialize reinforcement for floor boards by employing a simple structure.

CONSTITUTION: In a double floor unit whose floor board 2 is equipped with support legs 1 on its four corners or their vicinities, adhesive agent is injected, through an adhesive agent injecting port 3 provided to at least one location other than the four corners and their vicinities of the floor board 2, into a bellows 4 that is provided beneath the adhesive agent injecting port 3 and is in a length corresponding to the floor height. After the injection, the adhesive agent is hardened, and thereby the floor board 2 is stuck to a bed.