OPTICAL FIBER GYRO

PURPOSE: To provide a gyro having self-diagnostic function. CONSTITUTION: Light of a light source 1 is projected to a sensing coil 5 through a branched wave confluent element 2, interference light of clockwise light and anticlockwise light of the sensing coil 5 is taken out from the branched wave confluent element 1 so as to ad it to a photoelectric transfer circuit 7, the output Sa is input to an ac component detecting circuit 8e, and a light source driving circuit 8d controls light source driving current Sb so as to make a detected output Vdc constant. A signal corresponding to an input angular velocity is taken out from the Sa of the photoelectric transfer circuit 7 so as to output outward. At least either of a first signal S1 corresponding to the output of the alternating current detecting circuit 8e or the light source driving circuit 8d, or a second signal S2 which is generated by comparing the signal S2 with a lower limit value EL in a comparator 12, and inverting the polarity L→H (or the reverse) at exceeding the lower limit, is output outward as trouble diagnostic information.