PURPOSE: To enable the detection of high accuracy without being influenced by the intensity variation in a light source by deciding an angular velocity from the time upto the time when the output of the signal for rotary detection becomes zero from the time when a synchronous detection circuit detects the minimum value.

CONSTITUTION: The light emitted from a light source 1 circulates clockwisely and counterclockwisely the optical fiber 9 in a single mode with its bisection. Both lights returned to optical waveguide 3, 4 are then multipleped by a directional combining part 7, the interference light is emitted from the one end thereof through the optical waveguide 3 and converted into an electrical signal combined with a light detector 8. The output signals of the sine wave generating circuit 12 and superimposed wave generating circuit 13 composed by a waveform composing circuit 11 are impressed on a light phase modulator 10 as well. A synchronous detection circuit 14 takes a reference frequency component out of the output of the light detector 8 and then a zero point detecting circuit 1 switches over the gate signal g for a count motion to L→H at the time when the output of the circuit 14 becomes zero and switches over to H→L by the reset signal r transmitted from the circuit 13. The circuit 13 transmits the signal r of the time when its generating signal becomes zero to the circuit 15.