PURPOSE: To make it possible to detect phase difference twice the phase difference due to Sagnac's effect while minimizing the variation in the output of a gyroscope, by providing first and second laser beam distributor-couplers, a light propagation path forming apparatus, a frequency deviation apparatus and a photoelectric converter.

CONSTITUTION: Second and third laser beams forming fourth laser beam are respectively divided into two from the aspect of quantity of light by a half mirror to form fifth and sixth laser beams and the sixth laser beam receives diffraction by a frequency deviation element 8 and receives the same frequency deviation along with second and third laser beams to respectively form seventh and eighth laser beams. At this time, a frequency deviation apparatus allows the second and third laser beams forming the emitted fifth laser beam and the seventh and eighth laser beams forming the sixth laser beam to respectively propagate through the same light path in the frequency deviation apparatus. At this time, operation is performed so that the phase added during propagation in said apparatus becomes perfectly equivalent. By this method, if the interference beams of nineth and tenth laser beams emitted from a second laser beam distributor-coupler 13b are subjected to phase comparison, only phase difference twice the phase difference due to Sagnac's effect is obtained.