A polarization-maintaining optical fiber , comprising: a core ; a pair of stress-applying parts provided on both sides of the core and at point-symmetrical positions with respect to the core ; and a cladding surrounding the core and the stress-applying parts , wherein a diameter of the cladding is 125 μm, a diameter of each of the stress-applying parts is 33 to 37 μm, a distance between the pair of stress-applying parts is 8.6 to 15.4 μm, and a cut-off wavelength is less than or equal to 400 nm; characterized in that : the refractive index of the core is 1.456 to 1.458; the cladding comprises a first cladding , which directly surrounds the core and the stress-applying parts and is formed of fluorine-doped silica glass and a second cladding which is provided on an outer periphery of the first cladding and is formed of pure silica glass; a relative refractive index difference between the core and the first cladding is 0.35 to 0.45%; the first cladding has refractive index of 1.450 to 1.452; the second cladding has a thickness of 15 to 32.5 μm and refractive index of 1.456 to 1.458, and after applying a bending of 60 mm in diameter 10 times, a loss increase in a wavelength range of 400 to 680 nm is less than or equal to 0.1 dB and polarization crosstalk in the wavelength range of 400 to 680 nm is less than or equal to -30 dB. a core ; a pair of stress-applying parts provided on both sides of the core and at point-symmetrical positions with respect to the core ; and a cladding surrounding the core and the stress-applying parts , wherein a diameter of the cladding is 125 μm, a diameter of each of the stress-applying parts is 33 to 37 μm, a distance between the pair of stress-applying parts is 8.6 to 15.4 μm, and a cut-off wavelength is less than or equal to 400 nm; characterized in that : the refractive index of the core is 1.456 to 1.458; the cladding comprises a first cladding , which directly surrounds the core and the stress-applying parts and is formed of fluorine-doped silica glass and a second cladding which is provided on an outer periphery of the first cladding and is formed of pure silica glass; a relative refractive index difference between the core and the first cladding is 0.35 to 0.45%; the first cladding has refractive index of 1.450 to 1.452; the second cladding has a thickness of 15 to 32.5 μm and refractive index of 1.456 to 1.458, and after applying a bending of 60 mm in diameter 10 times, a loss increase in a wavelength range of 400 to 680 nm is less than or equal to 0.1 dB and polarization crosstalk in the wavelength range of 400 to 680 nm is less than or equal to -30 dB. the refractive index of the core is 1.456 to 1.458; the cladding comprises a first cladding , which directly surrounds the core and the stress-applying parts and is formed of fluorine-doped silica glass and a second cladding which is provided on an outer periphery of the first cladding and is formed of pure silica glass; a relative refractive index difference between the core and the first cladding is 0.35 to 0.45%; the first cladding has refractive index of 1.450 to 1.452; the second cladding has a thickness of 15 to 32.5 μm and refractive index of 1.456 to 1.458, and after applying a bending of 60 mm in diameter 10 times, a loss increase in a wavelength range of 400 to 680 nm is less than or equal to 0.1 dB and polarization crosstalk in the wavelength range of 400 to 680 nm is less than or equal to -30 dB. <EOS>