AbstractThe effect of polarization coupling which occurs in the coupler comprising the ring resonator for a fiber‐optic ring‐resonator gyro was studied. First, it is confirmed theoretically and experimentally that the resonant waveform in the ring resonator changes significantly by the polarization coupling. It is shown that the incline angle between the principal axes of two optical fibers causing the polarization coupling can be measured indirectly by observation of the resonant waveform.Next, three factors causing the drift are identified in the case of a fiber‐optic gyro using a ring resonator with polarization coupling. The magnitude of the drift for each factor is found quantitatively. As a result, it is found that the drift of about 10−7 ‐ 10−5 rad/s is generated in a typical case. Methods for reducing these drifts are presented. Finally, a rotation sensitivity of 1 × 10−5 rad/s with a time constant of 10 s is obtained in an improved fiber‐optic gyro.