A fiber optic gyro can overcome the defects encountered with conventional fiber optic gyros of phase-modulation method, closed-loop system with serrodyne modulation and digital modulation. A reference phase difference .DELTA..beta. and a ramp phase difference .sigma. are generated in an interference light intensity signal I by use of a triangular waveform, i.e., delta serrodyne waveform signal. The reference phase difference .DELTA..beta. is changed to constant values .DELTA..beta..sub.A and .DELTA..beta..sub.B whose absolute values are the same and whose signs are different at every times T.sub.A and T.sub.B. A phase x of the interference light intensity signal I becomes equal to x=.DELTA..theta.+.sigma.+.DELTA..beta.. The ramp phase difference .sigma. is controlled so as to satisfy .DELTA..theta.+.sigma.=0. Accordingly, at the stable point of the control loop, a Sagnac phase difference .DELTA..theta. is equal to the ramp phase difference .sigma.. Further, a phase x of the interference light intensity signal I does not contain the Sagnac phase difference .DELTA..theta., and hence x=.DELTA..beta.. Also, a fiber optic gyro can eliminate a bias, in particular, optically-generated bias. In a fiber optic gyro of phase-modulation method or delta serrodyne system, a phase controller is supplied with a phase control voltage signal V.sub.S of period T and a square wave signal V.sub.2 of period T for correcting an optical bias caused by an amplitude modulation generated by the phase control voltage signal V.sub.S in the added form. The square wave signal V.sub.2 has a constant magnitude H and is inverted in polarity at points in which the phase control voltage signal V.sub.S is maximized or minimized. The magnitude H and the polarity of the square wave signal V.sub.2 are selected such that an optical bias is minimized.