PURPOSE: To achieve a quality vibration detection even under an environment susceptible to a noise and an electromagnetic disturbance, by using an optical fiber for transmitting light between a photo branch and a vibration detector to convert physical vibration directly into an optical signal.

CONSTITUTION: Light returned to a photo branch 2 from a vibration detector 3 has a frequency of f+f0 obtained by adding a frequency of f0 based on a physical vibration to natural frequency f of light. The reflected light A' from the vibration detector 3 is focused to a branched light (B) to a light receiver from the light emitter 1 to be transmitted to a light receiver 4 through an optical fiber 7. The light receiver 4 has a photo heterodyne detector detects light with the frequency f0 from lights with the frequencies f and f+f0 received to obtain this as output. The light with the frequency f0 thus detected is converted into an electrical signal and displayed and recorded on a CRT and a recording paper.