We present a polarizing optical interferometer especially developed for force microscopy. The deflections of the force-sensing cantilever are measured by means of the phase shift of two orthogonally polarized light beams, both reflected off the cantilever. This arrangement minimizes perturbations arising from fluctuations of the optical path length. Since the measured quantity is normalized versus the reflected intensity, the system is less sensitive to intensity fluctuations of the light source. The device is especially well suited to static force measurements. The total rms noise measured is ≲0.01 Å in a frequency range from 1 Hz to 20 kHz.