An optical fiber sensing system comprising: a circulator that has first, second, and third ports, the first port being connected to a light source that outputs physical measurement light having a pseudorandom noise pattern; a first optical fiber that is connected to the second port of the circulator; at least one reflective sensor that is connected to an end of the first optical fiber, the reflective sensor returning light with a reflection spectrum obtained by modulating the physical measurement light in accordance with a change in physical quantity of a measurement target object to the first optical fiber; a light separating section that is connected to the third port of the circulator, the light separating section separating incoming light into two beams of light whose intensity changes complementarily in accordance with a change in the reflection spectrum of the return light from the reflective sensor; second and third optical fibers that are optical paths of the respective two beams of light coming from the light separating section; a fiber-optic delay line that is connected to the second optical fiber, the fiber-optic delay line adding, by means of an optical path difference, a predetermined time difference to one of the two beams of light separated by the light separating section; an optical coupler that couples an end of the third optical fiber with an end of the fiber-optic delay line; a light receiving section that receives the light from the optical coupler; and a detecting section that calculates amount of the physical change of the measurement target object on the basis of a ratio of the intensity of said one of the two beams of light and the intensity of the other. a circulator that has first, second, and third ports, the first port being connected to a light source that outputs physical measurement light having a pseudorandom noise pattern; a first optical fiber that is connected to the second port of the circulator; at least one reflective sensor that is connected to an end of the first optical fiber, the reflective sensor returning light with a reflection spectrum obtained by modulating the physical measurement light in accordance with a change in physical quantity of a measurement target object to the first optical fiber; a light separating section that is connected to the third port of the circulator, the light separating section separating incoming light into two beams of light whose intensity changes complementarily in accordance with a change in the reflection spectrum of the return light from the reflective sensor; second and third optical fibers that are optical paths of the respective two beams of light coming from the light separating section; a fiber-optic delay line that is connected to the second optical fiber, the fiber-optic delay line adding, by means of an optical path difference, a predetermined time difference to one of the two beams of light separated by the light separating section; an optical coupler that couples an end of the third optical fiber with an end of the fiber-optic delay line; a light receiving section that receives the light from the optical coupler; and a detecting section that calculates amount of the physical change of the measurement target object on the basis of a ratio of the intensity of said one of the two beams of light and the intensity of the other.