To directly and accurately detect fitting condition of a tire on a wheel during the traveling.

An optical fiber 11 wound around a rim part 31 of a wheel 3, on which a bead part 51 of a tire 5 is to be fitted, is formed with a Fiber Bragg Grating (FBG)11d to selectively reflect the light having a specified wavelength (Bragg wavelength) in the incident light to the optical fiber 11. A change (or the changed amount) of the Bragg wavelength of the reflected light by the FBG 11d or a change (or the changed mount) of the Bragg wavelength of the reflected wave relative to a default Bragg wavelength is detected by a light receiving/emitting unit 13. Based on a result of the detection, fitting condition of the bead part 51 of the tire 5 on the rim part 31 of the wheel 3 at a position where the FBG 11d is disposed, is detected.