PURPOSE: To improve insulation between the titled detector and an object to be detected and to miniaturize the apparatus by changing the quantity of optical attenuation of an optical fiber which forms coil-shaped part on its center according to the quantity of displacement of the object to be detected.

CONSTITUTION: A coil-shaped part 2 is formed on the center of an optical fiber 1, an illuminating part 3 consisting of an illuminating element LED such as an illuminating diode and an illumination control circuit L is connected to the one end of the coil-shaped part and a light receiving part 4 consisting of a light receiving element PD such as a photodiode and an amplifier AMP is connected to the other end of the coil-shaped part. The one end 2a of the coil-shaped part 2 of fiber 1 is a fixed end and an object to be detected 5 is fixed on the other end 2b. When the object to be detected is displaced in the direction of an arrow X, the shape of the coil-shaped part is changed by the displacement, the quantity of optical attenuation of fiber 1 is also changed and the light receiving part 4 generates the detected output vX corresponding to the extent of displacement. Thus, the insulation between the detector and the object to be detected can be improved and the apparatus can be also miniaturized.