PURPOSE: To eliminate fear of loosing adjusting function of a bolt by making the bolt having several screw thread sections, flat section, and plate receiving section from the upper end section in order rotatingly penetrate the screw section of a supporting plate whose root is thick and supporting it by means of a receiving plate.

CONSTITUTION: A bolt 1 is formed with two or three screw thread sections 11, flat section 15, and plate receiving section 14 from upper end side in order, and in addition, screw thread section 12 at the entire lower section or partially. This bolt 1 is threadedly supported by a fixing leg 4 on the floor surface and fixed by a nut member 3 after the adjustment of its height. In the next step, a supporting plate 2, whose root is thick and which has the screw hole engaging with the screw thread section 11, is made to rotatingly pass the screw thread sections 11 and be supported at the flat section 15 by means of the plate receiving section 14, to which the thick root 5 is fitted. A floor plate 6 is laid between this thick root 5 and the next thick root to form a double flooring, on which a finished plate is placed. The supporting plate 2, whose root is thick, is rotatably installed so that the height adjusting bolt 1 and supporting plate 2 may not fixed to each other because they are fastened due to screw. This leads to the elimination of the fear of loosing the height adjusting function of the bolt.