PURPOSE: To detect the displacement of a weight with high accuracy, by fixing an optical fiber to a fixing part and making it possible to directly detect the movement of the optical fiber due to the displacement of the weight by acceleration.

CONSTITUTION: The light from a light source 13 is reflected by a reflector 14 through a first optical fiber 11a and received by a light receiving part 15 through a second optical fiber 11b. Then, the light receiving part 15 is brought to a continuity and both terminals 20, 25 become almost equal in potential and potential difference is generated between both terminals of a resistor 24, that is, in a voltmeter 26. However, when acceleration of a definite value or more acts on a horizontal surface, the displacement quantity of a weight 12 becomes large and the curve of the optical fibers 11a, 11b becomes large while the light from the light source 13 passes through the optical fiber 11a to be reflected to a different direction by a light reflector 14 and no reflected light is incident to the optical fiber 11b and, therefore, no light reaches the light- receiving part 15 and the light receiving part 15 comes to a discontinuity and no potential difference is generated in the voltmeter 26. Therefore, when the potentials at both terminals of the voltmeter 26 become equal, it can be detected that the value equal to or more than acceleration predetermined at every direction has acted.