To provide a novel seating detecting device utilizing the bending of an optical waveguide such as an optical fiber.

A seating detecting device equipped on a seat 1 and having a permeation part 23 for permeating a light is provided with an optical waveguide such as an optical fiber 20 for changing the physical amount of a permeation light permeating the permeation part 23 by the change of the curvature of the permeation part 23 responding to a person's seating on the seat 1, a luminous means 3 for putting the light in the optical waveguide such as the optical fiber 20, a light receiving means 4 for receiving the light emitted from the optical waveguide and a detection means 5 for detecting the person' seating on the seat based on the received light in the light receiving means 4, when the physical amount of the permeation light permeated the permeation part 23 is changed by the change of the curvature of the permeation part 23 of the optical waveguide responding to the person's seated on the seat 1. A bending part can be formed in the optical waveguide such as the optical fiber 20.