To reduce the working cost of a hole formed to the frame of a wall panel, and to conduct bolting utilizing the hole surely.

A large number of holes 9, 9... for inserting bolts are disposed in a row to the flange 8 of a vertical frame material 3 in a square frame-shaped frame 2 installed onto the rear of a siding material 1 in the vertical direction at regular pitches. A large number of holes 12, 12... for inserting bolts are disposed similarly in a row to a web 11 in the vertical direction at regular pitches at places displaced from the holes 9, 9... of the flange 8 in the vertical direction.