A mechanism of phase separation in plasticized cellulose triacetate (CTA) films is investigated to produce a porous film that can be used in optical devices. Hot-stretched CTA films containing diisodecyl adipate (DIDA) show negative orientation birefringence with ordinary wavelength dispersion, as similar to that of the pure CTA film; the absolute value of the birefringence increases with wavelength. However, after extracting DIDA from the stretched films by immersion into an organic solvent, the birefringence of the film dramatically changes from negative to positive with extraordinary wavelength dispersion; the magnitude of the birefringence increases with the wavelength. This is an inevitable property for a multi-band quarter-wave plate. Numerous ellipsoidal pores with nano-scale diameter are detected by SEM observation in the film after the immersion, indicating that DIDA is segregated and formed ellipsoidal domains in the CTA matrix during annealing and stretching. These results indicate the great contribution of form birefringence originated from anisotropic pores, and that the combinations of orientation birefringence from CTA and form birefringence from pores give extraordinary wavelength dispersion. Furthermore, annealing time and stretching condition affect the phase separation, i.e., the shape and size of dispersed domains. Light transmittance of CTA/DIDA decreases with the annealing time, as compared with that of pure CTA. It suggests that DIDA is segregated from CTA and forms domains during heating. Birefringence of the extracted CTA/DIDA films increases with the strain rate, which is attributed to the enhanced form birefringence by the prolonged voids. The high level of chain orientation of CTA is responsible for the large aspect ratio of the voids.