Passive resonator gyro comprising an injection laser emitting an initial optical beam at a first frequency and a cavity, characterized in that the gyro includes: - an optical splitter making it possible to divide the initial optical beam into a first optical beam, into a second optical beam and into a third optical beam, the first beam being injected into the cavity in a first direction, the second and third optical beams being injected into the cavity in the opposite direction; - a first slaving device (10, 12, 13, 14) making it possible to slave the first frequency of the first optical beam to a first proper frequency corresponding to a first mode of resonance of the cavity or to slave a first proper frequency corresponding to a first mode of resonance of the cavity to the first frequency of the first optical beam; - a second slaving device (20, 22, 23, 24, 25) making it possible to slave the second frequency of the second optical beam to a second proper frequency corresponding to a second mode of resonance of the cavity; - a third slaving device (30, 32, 33, 34, 35) making it possible to slave the third frequency of the third optical beam to a third proper frequency corresponding to a third mode of resonance of the cavity, the three proper frequencies being all different from each other; - means for measuring a first frequency difference between the first frequency and the second frequency and for measuring a second difference between the second frequency and the third frequency, these two frequencies combined together being representative of the length of the cavity and the angular rotational velocity of the cavity according to an axis perpendicular to the median plane of said cavity. - an optical splitter making it possible to divide the initial optical beam into a first optical beam, into a second optical beam and into a third optical beam, the first beam being injected into the cavity in a first direction, the second and third optical beams being injected into the cavity in the opposite direction; - a first slaving device (10, 12, 13, 14) making it possible to slave the first frequency of the first optical beam to a first proper frequency corresponding to a first mode of resonance of the cavity or to slave a first proper frequency corresponding to a first mode of resonance of the cavity to the first frequency of the first optical beam; - a second slaving device (20, 22, 23, 24, 25) making it possible to slave the second frequency of the second optical beam to a second proper frequency corresponding to a second mode of resonance of the cavity; - a third slaving device (30, 32, 33, 34, 35) making it possible to slave the third frequency of the third optical beam to a third proper frequency corresponding to a third mode of resonance of the cavity, the three proper frequencies being all different from each other; - means for measuring a first frequency difference between the first frequency and the second frequency and for measuring a second difference between the second frequency and the third frequency, these two frequencies combined together being representative of the length of the cavity and the angular rotational velocity of the cavity according to an axis perpendicular to the median plane of said cavity.