We investigated the segmental motion of supercooled random copolyester by elastic neutron scattering. Neutron polarization analysis was applied in order to decompose the total scattering into the coherent and incoherent contributions, which plays an essential role in the careful discussion of the scattering vector dependence of the elastic incoherent scattering. The study on highly oriented glassy random copolyester revealed the anisotropic features of segmental motions in a wide temperature region from glassy to liquid states. In this paper, we report in detail (i) the anharmonic increase of mean-square displacement, (ii) the non-Gaussian parameter A(T), and (iii) the elastic incoherent structure factor for the rotational segmental motion.