PURPOSE: To modify the interior of walls, ceiling surfaces, etc., rapidly at low cost.

CONSTITUTION: At an end of a mounting surface 20, a gap 27 is provided between said end and a surface 25 normal to the surface 20 and a plurality of fixing devices 1 having a locking groove 3 extending in the londitudinal direction are fixed in advance to the mounting surface of a frame of building, thereby providing an insertion opening 8 on the extended line. The periphery of a square panel is bent inwardly to form a box-like interior panel 10 having a locking section 12. Each locking section 12 of the panels 10 is inserted into each groove 3 of the fixing devices 1 from the opening 8 in succession so that the panels 10 are fixed to the surface 20.