PURPOSE: To cause quick response to the change in rotation and prevent the error of a ring laser unit, by straightly vibrating light detectors and Fresnel-effect-producing transparent members 15 the same phase as the displacement of an interference fringe.

CONSTITUTION: A laser tube 4 is provided among reflectors 1, 2, 3 so that a ring laser unit is formed. Output beams 5, 6 are mixed with each other by a light mixer 7. An interference fringe is enlarged by a columnar lens 12. Light detectors 8, 9 and transparent members 13, 14 such as quartz, which act to produce a vibration bias by the Fresnel effect, are secured on a vibratory base 15. The direction and frequency of movement of the interference fringe are detected by the light detectors to generate a beat signal corresponding to the angular velocity and direction of rotation of the ring laser unit to straightly vibrate the base 15 at the same phase and amplitude as the movement of the interference fringe. This results in enabling quick response to the change in the rotation and the vibration of only a portion of small weight. Therefore, the error and fatigue of the ring laser unit can be provented.