A displacement detection device comprising: a low-coherence laser light source for emitting light and having a plurality of oscillation spectra associated with a length of a resonator; a lens for focusing light from the laser light source ; a polarization maintaining fiber for transmitting light focused by the lens , and having a length L fiber that satisfies L fiber ≠ 2 m n eff L cav L beat / λ , where m is an integer equal to or larger than 0, L cav is the length of the resonator, n eff is a refractive index of the resonator, L beat is a beat length obtained from a difference between propagation constants of two polarization modes of the polarization maintaining fiber , and λ is a wavelength of the light source , wherein the length L fiber is selected to lie between (2 m n eff L cav L beat )λ, and (2 (m+1) n eff L cav L beat )/λ ; and a displacement detector for dividing a beam transmitted by the polarization maintaining fiber into two beams and for converting respective beams to an interference signal by causing the respective beams to interfere with each other, in order to set a change of phase applied to the respective beams to an amount of displacement, wherein the polarization maintaining fiber includes a plurality of polarization maintaining sub-fibers fitted to each other in series by removable connectors, and when a plurality of fitting portions exists, in order of connection of the polarization maintaining sub-fibers , a total length of any combination of the polarization maintaining sub-fibers including a last polarization maintaining sub-fiber and any combinations of other polarization maintaining sub-fibers except the last polarization maintaining sub-fiber satisfies the above condition for L fiber ; characterized in that when fitting the polarization maintaining sub-fibers , X and Y polarization mode axes of one polarization maintaining sub-fiber are connected to different X and Y polarization mode axes of another polarization maintaining sub-fiber to be connected. a low-coherence laser light source for emitting light and having a plurality of oscillation spectra associated with a length of a resonator; a lens for focusing light from the laser light source ; a polarization maintaining fiber for transmitting light focused by the lens , and having a length L fiber that satisfies L fiber ≠ 2 m n eff L cav L beat / λ , where m is an integer equal to or larger than 0, L cav is the length of the resonator, n eff is a refractive index of the resonator, L beat is a beat length obtained from a difference between propagation constants of two polarization modes of the polarization maintaining fiber , and λ is a wavelength of the light source , wherein the length L fiber is selected to lie between (2 m n eff L cav L beat )λ, and (2 (m+1) n eff L cav L beat )/λ ; and a displacement detector for dividing a beam transmitted by the polarization maintaining fiber into two beams and for converting respective beams to an interference signal by causing the respective beams to interfere with each other, in order to set a change of phase applied to the respective beams to an amount of displacement, wherein the polarization maintaining fiber includes a plurality of polarization maintaining sub-fibers fitted to each other in series by removable connectors, and when a plurality of fitting portions exists, in order of connection of the polarization maintaining sub-fibers , a total length of any combination of the polarization maintaining sub-fibers including a last polarization maintaining sub-fiber and any combinations of other polarization maintaining sub-fibers except the last polarization maintaining sub-fiber satisfies the above condition for L fiber ; characterized in that when fitting the polarization maintaining sub-fibers , X and Y polarization mode axes of one polarization maintaining sub-fiber are connected to different X and Y polarization mode axes of another polarization maintaining sub-fiber to be connected. <EOS>