A holographic method for reconstructing the polarization of light from a photoelastic model is presented. Three typical patterns associated with the principal stresses, that is, Nisida and Saito's pattern in a Mach-Zehnder inter-ferometer, Fourney and Mate's pattern in double-exposure holographic inter-ferometry and dark-field isochromatics in conventional photoelasticity, are obtained from one double-exposure hologram by observing the reconstructed image having information of the original model's polarization. Though the experimental condition is more stringent than that of Fourney and Mate's method, the intensity pattern obtained here is simpler than theirs. The present method can completely separate dark-field isochromatics from other patterns and does not require an optically flat and parallel plane model. The analytical description of the method and the experimental verification are described.