The intrinsic birefringence can be approximately represented by the following formula. ##EQU1## wherein ?? is the circular constant, d is a density, N is the Avogadro number, n is an average refractive index, and ????=??11 -????? (wherein ??11 is a polarizability per monomer in the direction of the molecular chain axis of the polymer and ????? is a polarizability in the direction perpendicular to