The present invention relates to a fiber-optic gyrometer including a Sagnac interferometer using two light waves propagating in opposite directions in a interferometer loop including a photodetector which delivers an electrical signal Ud representing the light intensity of the interference between the two waves, and for optically phase-shifting the waves controlled by a square-wave modulation signal Um suitable for controlling an optical phase variation at a frequency FO substantially equal to 1/(2t.sub.o), where t.sub.o is the propagation time of a wave through the guide. The photodetector is connected to at least a first sampling circuit and a second sampling circuit which are controlled in phase opposition by a clock. The gyrometer includes a means for inverting the phase of the clock at a frequency f, which is very much less than the frequency FO.