The plurality of detected portions (32-1 to 32-m) are formed in substantially the same position in the longitudinal direction of the light guide member (16), for example, in different directions in the circumferential direction. Each detected portion changes the optical characteristic of the sensor light from the light source (12) guided by the light guide member according to the amount of curvature of the light guide member, whereby different absorption wavelength characteristic regions And an optical characteristic changing member that generates an optical signal having a wavelength. The optical characteristic changing member is a member made of metal particles that absorbs light in a predetermined wavelength range and has a special spectral absorption spectrum different from the spectral absorption spectrum peculiar to the metal. Optical characteristic change members having different spectral absorption spectra different from each other are arranged in the plurality of portions to be detected, and the special spectral absorption spectra of all the optical characteristic change members are determined by the emission wavelength region of the sensor light and at least a part Overlap each other.複数の被検出部(32-1~32-m)は、例えば導光部材(16)の長手方向で略同じ場所に周方向で異なる向きに形成される。各被検出部は、導光部材の湾曲量に応じて、導光部材が導光する光源(12)からのセンサ光の光学特性を変化させることで、被検出部毎に異なる吸収波長特徴領域を有する光信号を生成する光学特性変化部材を含む。光学特性変化部材は、所定の波長域の光を吸収する、金属粒子からなる部材であり、当該金属固有の分光吸収スペクトルとは異なる特殊分光吸収スペクトルを有する。複数の被検出部には、互いに異なる特殊分光吸収スペクトルを有する光学特性変化部材が配置されており、全ての光学特性変化部材の特殊分光吸収スペクトルは、センサ光の発光波長領域と、少なくとも一部で重なりを有している。
