PURPOSE: To prevent propagated light from having the loss of its quantity at a beam splitter and to secure high accuracy by providing two mode filter fibers of the same length where light emitted by a light emitting element is branched into two and coupled with both terminals of a sensor coil.

CONSTITUTION: A light beam which is sent by a light emitting element 30 and branched by a half-mirror 36 is coupled with one-terminal sides of mode filter fibers 40A and 40B and only laser light in single mode is outputted from the other terminal sides of the fibers 40A and 40B. Then, the light is branched into two by a half-mirror 44 and coupled with both ends of an optical fiber 50. The beam has a phase difference at the part of the sensor coil 52 which is rotated and is phase-modulated by a part 56 wound around 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 and multiplexed by a mirror 44, further branched, and coupled with the other-terminal sides of the fibers 40A and 40B. This beam is made incident on a light receiving element 50 through a mirror 36.