Sensing parts are formed in different directions in a circumferential direction in substantially a same position in the longitudinal direction thereof. Each sensing part is configured to include an optical characteristic changing member which generates the optical signals having absorption wavelength characteristic regions that vary from sensing part to sensing part by giving an optical characteristic change, which differs from that of other sensing parts, to the sensor light incident thereon in accordance with an amount of bending in a specific direction. A light detector detects the optical signals included in sensor light from a light source, which has passed through the sensing parts and undergone the optical characteristic change.