A bending detecting system includes a light guide, a first grating and a light detector. The light guide has elongated shape and is configured to guide an incident light in a propagating direction. The light guide includes a core and a cladding disposed around the core. The first grating is disposed in a boundary area, the boundary area including an outer surface of the core, and an adjacent area that is adjacent to the outer surface. The first grating includes a first periodic structure along the propagating direction with a first pitch, and is configured to generate a first diffracted light from the incident light. The light detector is configured to detect the first diffracted light from the first grating, and detect a bending of the light guide based upon an optical feature amount of the first diffracted light.
