PURPOSE: To facilitate the bonding of a rib on the rear side of a floor pan and a bath tub on a floor surface.

CONSTITUTION: A bond holder 5 with a roughly U-shaped cross section comprising a mesh is fitted into ribs 1A and 1B. The bond holder 5 is filled with a bond 6 beforehand. The bond holder 5 is pressed onto the floor surface and the ribs 1A and 1B are erected on the floor surface extruding the bond 6 from the bond holder 5. Thus, the bond 6 can be applied in the perimeter of the ribs 1A and 1B and the ribs 1A and 1B are bonded strongly on the floor surface with a sufficient amount of the bond 6 thereby preventing the waste of the bond.