At least one optical fiber of a fiber sensor has sensing parts whose number is number of the anticipated inflection points+one or more, the anticipated inflection points being inflection points of a shape of a detection target range of a detection target. The number of the anticipated inflection points is decided based on one of a functional limit and a structural limit which limit a degree of freedom in a bending shape of the detection target. A space L1 between the anticipated inflection points is L1=r1·θ1, wherein r1 is a curvature radius at a maximum bending of the detection target range of the detection target, θ1 is a central angle of an arc created by the space between the anticipated inflection points at the maximum bending, and θ1≧π/2.
