A new instrument for measuring small angina by use of multiple total internal reflections in heterodyne interferometry is presented. With this instrument we can achieve a small rotation angle only by measuring the variation in phase difference between s- and p-polarization states. To improve its sensitivity we increase the number of total internal reflections by using two parallelogram prisms instead, of two right-angle prisms. The angular resolution of the new instrument is better than 2.2 × 10-6 rad over the measurement range -2.12° ≤ θ ≤ 2.12° for 20 total-internal reflections. The experimental results and the theoretical curve are in good agreement.