PURPOSE: To obtain a gyro whose accuracy is not affected by variation in the ampli tude of propagated light by providing a controller which detects a frequency component which is twice as high as a phase modulation frequency in the output of a light receiv ing element and controls a phase modulator so that the component is eliminated.

CONSTITUTION: A transmitted light beam from a light emitting element 30 which is branched by a half-mirror 32 is further branched by a half-mirror 38 and coupled with both ends of an optical fiber 40. This beam has a phase difference at the part of a sensor coil 42 and is phase-modulated 44. Thus, the clockwise and counterclock wise light beams are outputted from both ends of a fiber 40, multiplexed by a mirror 38, and incident on a light receiving element 48 through a mirror 32. The output of this element 48 is passed through a preamplifier 50 and detected synchronously by a synchronous detector 54 which receives the angular frequency ωm of the modulator 44 from an oscillator 56. The output passed through the amplifier 50 is detected synchro nously by a synchronous detector 60 with an angular frequency 2ωm from a frequency multiplier 58 and compared with a zero level by a comparator 62, and an oscillator 56 makes an adjustment with the amplitude of the angular frequency ωm of the modula tor 44 so that the input difference signal is zero.