PURPOSE: To facilitate wiring in a panel main body by putting an insulation material inside the panel main body formed with a surface material mounted on the front and rear surfaces of a panel frame and installing a recessed groove for the wiring to the insulation material.

CONSTITUTION: An interior finishing material 7 is mounted to either the front surface or the rear surface of a panel frame 1 as a surface material 2 where expanded phenol 13 is injected into the whole rear surface of the interior finishing material 7. The expanded phenol 13 is pressed by a roller 14 and the portion pressed by the roller 14 is pressed by a press 15 or the like, thereby forming a rectangular groove 5 on the insulation material 4. Then, an exterior finishing material 6 is mounted to the other front surface or the rear surfaces of the panel frame 1 as the surface material 2, thereby forming a wall panel A. It is, therefore, possible to wire cables or the like inside the wall panel, using the rectangular groove 5.