PURPOSE: To measure an accurate angular velocity over a wide range consisting of measurable ranges of a low-speed and a high-speed detection sensor by providing a low-speed and a high-speed sensor coil which have a phase modulating element in the middle and output light from both ends.

CONSTITUTION: A light beam from a light emitting element 30 is split by a half-mirror 36 into two, which are passed through lenses 38A and 38B; and only single-mode laser light beams are sent from single-mode optical fibers 40A and 40B to half-mirrors 44A and 44B. Light beams which are each split into two are inputted to a high-speed optical fiber 50A and a low-speed optical fiber 50B and phase-modulated by optical fiber phase modulation parts 56A and 56B. The phase-modulated light beams enter a photodetecting element 60 through a lens 58 and the output of the element 60 is inputted to synchronous detectors 64A and 64B. The detectors 64A and 64B output voltage signals indicating phase differences generated at coils 52A and 52B. Outputs of the detectors 64A and 64B are monitored by comparators 72 and 74 and controlled by a changeover switch 68, so a correct rotating speed is known.