PURPOSE: To prevent propagated light from having the loss of its quantity at a beam splitter and to secure high accuracy by providing asymmetrical X type four-terminal branching and mixing elements as branch elements which couple projection light from a light emitting element with both ends of a sensor coil.

CONSTITUTION: The light beam from the light emitting element 30 after being transmitted through a half-mirror 30 is passed through a mode filter fiber 40, branched into two by an asymmetrical X type four-terminal branching and mixing element 44, and coupled with both ends of an optical fiber 50. This beam has a phase difference at the part of the sensor coil 52 and is phase-modulated by a phase modulating element 54 which is driven with an alternating current from a phase modulating element driving circuit 68. Thus, the clockwise and counterclockwise light beams are outputted from both ends of the fiber 50, mixed by the element 44, and further coupled with the other end of the fiber 40. This beam is incident on a light receiving element 60 through a mirror 36 and its output is inputted a synchronous detector 64 which receives the driving angular frequency of the element 54 from an oscillator 66, and a low-pass filter 70 extracts a signal proportional to the rotating angular speed from the output and outputs the signal.