PURPOSE: To reinforce rigidity and enhance the quality of an outside view by installing an offset havine a specified height to at least one of wall panel front and rear surfaces in the longitudinal direction in parallel

CONSTITUTION: A sheet metal 2 comprises a front side part which includes a front surface 2a and a side surface 2c and a top surface 2d and a bottom surface 2e and a rear side part which includes a rear surface 2b and a side surface 2c and a top surface 2d and a bottom surface 2e. The two front and rear parts are assembled in the shape of a box where any of the front and rear parts are produced by bending one sheet metal with a press or the like, thereby shaping up the sheet panel 2. Foam resin, such as urethane is interposed in the box-shaped sheet panel 2, thereby forming a wall panel 1. A plurality of wall panels 2 are laminated in the vertical direction where the front surface 2a and the rear surface 2b are adapted to be a vertical wall surface. Portions extending forward by an offset 4 having a specified height (h) are installed to at least one of the front surface 2a and the rear surface 2b of the wall panel 1 in the longitudinal direction in parallel. Projected and recessed parts 5 and 6 are installed to the top surface 2d and the bottom surface 2e. When the lower-sided recessed part 5 is placed on the top of the upper-sided projected part 6, large deviation is eliminated by way of seal tape 7.