A two-dimensional phase-detection system for a surface-plasmon-resonance sensor is presented. The sensor utilizes polarization interferometry to detect phase differences between the s and p polarizations. We successfully detected a spatial phase-difference variation, resulting from the biomolecular interactions, of less than $1\times 1{\mathrm{mm}}^2$ . The phase stability demonstrated in the experimental results was approximately 0.09°, and the corresponding change in the refractive index detection limit was approximately $4.3\times {10}^{-6}$ . The common-optical-path configuration of the proposed method allowed us to reduce disturbances from ambient conditions. Furthermore, this method is capable of real-time array detection.