This paper introduces a new interferometer for measuring the surface roughness based on the principle of differential polarization interferometry. A Faraday modulating system is used to modulate the phase between two interference light beams and the phase is detected by phase-locked interference. It has excellent stability even under normal condition, and rapid and nocontact measurement can be done without a special reference surface. The instrument can produce the surface profile and statistical data with height sensitivity 2 nm and lateral resolution 1.2 μm.