A curvature sensor includes a light source, a flexible light guide including cores, FBG sensors that are provided in the cores and constitute FBG sensor groups at predetermined positions along longitudinal axes of the cores. The curvature sensor includes a detector that detects an optical spectrum of light from the FBG sensors, and a processor that determines a bend position and a bend amount of the light guide. The FBG sensor groups include a first FBG sensor group and a second FBG sensor group next to it. FBG sensors in the first and second FBG sensor groups include gratings having first and second pitches. The first pitch is shorter than the second pitch and is closer to the second pitch than other pitches of gratings of FBG sensors that include gratings having pitches shorter than the second pitch.