PURPOSE: To surely know deformed state of a very long structure with one optical fiber by arranging the optical fiber along the longitudinal direction of the coil spring for detecting light quantity changes of the optical fiber caused by contraction/expansion of a coil.

CONSTITUTION: A sensor is constituted by arranging an optical fiber 12, at both top and bottom, on the outside surface of a coil spring 11 along its longitudinal direction. It is coated with a coating film 13, consisting of a heat shrinkable tube 14 and an elastic Si-group coating film 15 that covers the outer surface of 13, for fixing. On the outside of the coating film 13, a U-shaped beam 16 made of, for example, rubber is fixed with an Si-group filler material 17 in between, so that the beam 16 is adhered to the surface of structure for fixing. If the coil 11 shrinks due to deformation of the structure in the longitudinal direction, the pitch intervals of the coil 11 become small, causing local bending of the fiber 12. Therefore, quantity of light which comes in from one end of the fiber 12 and comes back, changes. From this light quantity change, compression amount of a structure is measured.