A polarization-maintaining optical fiber of the invention includes: 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. The maximum refractive index of the core is greater than each of maximum refractive indexes of a first cladding coat, a second cladding coat, and a third cladding coat. The 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. The 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.