PURPOSE: To achieve the improvement in sensitivity and reduction in troubles by disposing a fiber element bundle to photo detecting part.

CONSTITUTION: A photo detector 151 is constituted by one end of the fiber cable bundle 15 connected to a photoelectric converter 16 with respect to the condensing optical system 131 mounted to one end of one fiber cable 13 connected to an optical branching filter 12. Then, when earthquake occurs, the light beam focussed at the condensing optical system 131 in the earthquake sensor 14 causes a position deviation and enters the position deviated from the center of the photo detecting part 151. It is then converted to an electric signal by a photoelectric transducer 16, after which the electric signal is subjected to specified processing in a signal processing part 17 and is then inputted to a central processing unit 18. Since the photo detecting part 151 is constituted by the section of the fiber cable bundle 15 comprising disposing a multiplicity of fiber cables of a micro diameter in concentrical circular form, the sensitivity is increased and in addition, troubles are less because there are no mechanical moving parts.