As the copolymer, an MMA/styrene/maleicanhydride terpolymer can be used as heat-resistant PMMA.It is also desirable that the amounts of an ultraviolet absorber, a dye, a pigment and other additives incorporated be small.In order to attain a biaxially orienting effect sufficiently by sufficient entanglement of polymer chains, it is indispensable that the weight average molecular weight of PMMA used