An optical fiber includes: a core (1) having an outer diameter (D1) of greater than or equal to 8.2 μm and less than or equal to 10.2 μm; a first cladding (2) surrounding the core (1) and having an outer diameter (D2) of greater than or equal to 30 μm and less than or equal to 45 μm; a second cladding (3) surrounding the first cladding (2) and having a thickness (T) of greater than or equal to 7.4 μm; and a support layer (4) surrounding the second cladding (3). The relative refractive index difference which is the ratio of the difference between the refractive index of the support layer (4) and that of the second cladding (3) to the refractive index of the support layer (4) is greater than or equal to 0.5%.