To prevent generation of mold and improve workability by temporarily fixing and connecting an unevenness adjusting bracket to a bed material by a drying method.

Foam urethane having a heat insulating effect is sprayed to a wall surface 11a to form an urethane layer 12. Secondly, a linear groove 12a is cut for inserting both tips 13b in the cross direction of a bed material 13. Subsequently, an unevenness adjusting bracket 19 is temporarily fixed to the bed material 13 by screws or a magnet, the tip 13b of the bed material 13 is put in the V-shaped linear groove 12a, and the upper and lower tips of the bed material 13 are aligned with a marking line L to temporarily fix the bed material by a screw mechanism 19a of the unevenness adjusting bracket 19. Hard urethane is sprayed between the linear groove 12a and the tip 13b of the bed material 13 to form a fixed urethane layer 14 by adhesion, and when the layer 14 is solidified, the unevenness adjusting bracket 19 is removed. Thus, once leveling is enough. A wall board is screwed on the fixed bed material by a tapping screw.