PURPOSE:To obtain accurate angular speed information without noise influence by a method wherein a reference signal for synchronously detecting a basic wave component is obtained by dividing into two a secondary higher harmonic wave. CONSTITUTION:A light emitted from a light emitting element 10 is transmitted through a sensor coil 20 into a light receiving element 26, where it is square-law- detected and amplified 22. Of said amplified signal, only basic wave component P (DELTAtheta, t) of an angular frequency OMEGA is permitted through a filter 36 and synchronously detected 34. Of an amplified signal 32, a secondary higher harmonic wave is selected by a filter 37, shaped into a rectangular wave in a comparator 100 and 1/2-divided 42. In other words, a counter 102 converts oscillation 2OMEGA into an oscillation OMEGA. Then an angular frequency of a phase modulation drive circuit 22 makes oscillation OMEGA into a rectangular wave at a comparator 101 and inputs it into a clear terminal of a comparator 102, which is cleared at every frequency of phase modulation. Then a phase correction circuit 40 gives a phase difference PSI' to the output of a divider 42, permitting a reference wave P (DELTAtheta, t) and reference signal w(t) free of phase difference to be obtained.
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