To form a back cover area into a smooth bending face and classify and thrown away a binding tool by fixing the binding tool fixed from inside by a screw.

A back round component 16 with the transverse face shape forming a half arc shape is fixed outside or inside a back cover 12 through a means of bonding or the like. The back round component 16 is provided with a thick wall section 16A forming a thick wall central section, and screw channels 16B are formed on two areas on the thick wall section 16A. A leg component 25 of a binding tool 17 is positioned on the position coaxial to the screw channels 16B, and a screw 30 inserted through from above the leg component 25 is clamped and fixed on the screw channels B to fix the binding tool 17.