PURPOSE: To prevent cracking by bending from occurring, and provide efficient antistatic effect by a method wherein two kinds of resin sheets made by elastically plasticizing polystyrene resin are laminated together.

CONSTITUTION: A resin sheet A is such that is made by blending shock-resistant polystyrene resin in 18-35 pts.wt., crystalline polystyrene resin in 15-30 pts.wt., rubber denatured block elastomer in 34-47 pts.wt., and thermoplastic elastomer in 2-9 pts.wt.. A resin sheet B is such that is made by blending shock-resistant polystyrene resin in 18-35 pts.wt., crystalline polystyrene resin in 13-25 pts.wt., rubber denatured block elastomer in 30-55 pts.wt., thermoplastic elastomer in 1-7 pts.wt., and antistatic agent in 1-7.5 pts.wt.. A cover 1 is made up with the resin sheet B put inside, and plasticity of elastomers absorbs and disperses, working together with undercuts 6 provided to corners 4 of the cover, stresses applied to a fitting part 9 of the cover.