A polarization-maintaining optical fiber comprising: a core; a pair of stress-applying parts disposed at both sides of the core at a distance; and a cladding coat that surrounds the core and the paired stress-applying parts, wherein the cladding coat comprises: a first cladding coat disposed around the core; a second cladding coat disposed around the first cladding coat; and a third cladding coat disposed around the second cladding coat, and wherein a maximum refractive index of the core is greater than each of maximum refractive indexes of the first cladding coat, the second cladding coat, and the third cladding coat, a maximum refractive index of the second cladding coat is lower than each of maximum refractive indexes of the first cladding coat and the third cladding coat, a coefficient of thermal expansion of each of stress-applying parts is greater than a coefficient of thermal expansion of the cladding coat, each stress-applying part is provided to cut the second cladding coat at a position in a circumferential direction, and each stress-applying part is provided to protrude in a radial-outer direction from a boundary between the second cladding coat and the third cladding coat. a core; a pair of stress-applying parts disposed at both sides of the core at a distance; and a cladding coat that surrounds the core and the paired stress-applying parts, wherein the cladding coat comprises: a first cladding coat disposed around the core; a second cladding coat disposed around the first cladding coat; and a third cladding coat disposed around the second cladding coat, and wherein a maximum refractive index of the core is greater than each of maximum refractive indexes of the first cladding coat, the second cladding coat, and the third cladding coat, a maximum refractive index of the second cladding coat is lower than each of maximum refractive indexes of the first cladding coat and the third cladding coat, a coefficient of thermal expansion of each of stress-applying parts is greater than a coefficient of thermal expansion of the cladding coat, each stress-applying part is provided to cut the second cladding coat at a position in a circumferential direction, and each stress-applying part is provided to protrude in a radial-outer direction from a boundary between the second cladding coat and the third cladding coat. a first cladding coat disposed around the core; a second cladding coat disposed around the first cladding coat; and a third cladding coat disposed around the second cladding coat, and wherein a maximum refractive index of the core is greater than each of maximum refractive indexes of the first cladding coat, the second cladding coat, and the third cladding coat, a maximum refractive index of the second cladding coat is lower than each of maximum refractive indexes of the first cladding coat and the third cladding coat, a coefficient of thermal expansion of each of stress-applying parts is greater than a coefficient of thermal expansion of the cladding coat, each stress-applying part is provided to cut the second cladding coat at a position in a circumferential direction, and each stress-applying part is provided to protrude in a radial-outer direction from a boundary between the second cladding coat and the third cladding coat.