PURPOSE: To form an oriented film at a low temp. by using specific polyimide amide as a material for the oriented film of a plastic base plate.

CONSTITUTION: A polymide amide polymer contg. the trimellitic acid anhydride expressed by the formula ( I ) (X is an aliphat. or hetero atom or an aliphat. group contg. hetero atom, more specifically -COO-, -CO-, -CH2-, -C(CH3)2-, -S-, etc.) as a main constitutional unit is used as a material for the oriented film of a liquid crystal display element formed by placing plastic base plates which have patterned transparent electrodes and are subjected to a polar orientation treatment to face each other, sealing the periphery thereof and filling a nematic liquid crystal therein. The soln. is coated on the base plate and the oriented film is formed at a relatively low temp. (for example, about 120°C, 60min). The refractive index is about 1.70W1.78.