PURPOSE: To obtain high detection sensitivity by controlling a phase modulator with the output of at least either of the 1st and the 2nd photodetectors and setting the operation point of a ring interferometer, and detecting an angular speed of rotation from the difference output of the two photodetectors.

CONSTITUTION: The sum output 329 of the 1st and the 2nd photodetectors 321 and 322 is held invariably constant under the control of the 1st and the 2nd driver circuits 327 and 328. At this time, the difference output 325 of the 1st and the 2nd photodetectors 321 and 322 is zero unless the ring interferometer rotates or nearly proportional to the angular velocity of rotation of the ring interferometer when the interferometer rotates. For the purpose, the difference output 325 is measured to detect the angular velocity of rotation of the ring interferometer.