PURPOSE: To improve the measurement precision of a physical quantity such as a rotary angular velocity by branching the optical path of a light source into two and forming a two-system interference part, performing photoelectric conversion of output light, and calculating the mean value of electrical signals.

CONSTITUTION: Light from the light source 21 is converted by a lens 22 and made incident on an optical fiber branching device 23, and the light is split equally and guided to optical fiber type directional couplers 24 and 25. Directional output light beams from the couplers 24 and 25 are made incident on both ends of optical fiber coils 26 and 27 which are made of cores of a two-core flat cable and equal in length at the same time. Outputs light beams from the coils 26 and 27 are made re-incident on the couplers 24 and 25 and their interference light beams are made incident on photodetectors for photoelectric conversion, thereby calculating the mean value of two electric signals. Consequently, the output light of the two-system interferometer is reduced in variation on the whole because of no correlation of light fluctuations, so the measurement precision of physical quantities such as a rotary angular velocity is improved greatly.