PURPOSE: To make the measurement of a pressure applied on a spring easy, to make the structure rigid and to make manufacture easy, by making a buffer spring receiving a compressing force to be a coil spring, which is formed by winding a metal pipe, in the hollow part of which an optical fiber is provided, in a coil shape.

CONSTITUTION: In a hollow part of a filling pipe 1, an optical fiber 3 is supported by a protecting pipe 4. The space between the protecting pipe 4 and the inner surface of the filling pipe 1 is filled with fine powder 5. The filling pipe is formed in a shape having two coils that are connected by a straight-line shaped central part 11. Both terminal parts 12 and 13 are connected to a transmitter and a receiver through terminal 21 and 22. Light having a specified wavelength is transmitted from the transmitter and the light, whose wavelength is changed by a compressing force applied on the coils, is received by the receiver. Thus the compressing force is detected.