To enable an intermediate panel 20 to be erected without any accidental sideways slippage even when the intermediate panel 20 is connected with a back panel 10 and a top board 2 at fewer locations.

The rear end upper section of the intermediate panel 20 is fitted in a groove section 16 which is formed on the top beam 12 in the back panel 10 to extend in the lateral direction through a sliding block which is freely movable along the front surface of the back panel. The front end lower section of the intermediate panel 20 is fixed through a clamp body 50 which is fitted on the front edge section of the top board 2. A friction means 39 having an attaching body with a friction plate provided on the lower surface thereof is freely detachably fitted in a downward open groove of a skirting board 23 which is the lower end section of the intermediate panel. In this way, the sideways slippage of the intermediate panel 20 is prevented by the friction plate brought into contact with the top board 2.