An optical fiber line comprising: a first optical fiber; and a second optical fiber having an end connected to an end of the first optical fiber by fusion-splicing, wherein the first optical fiber is mainly comprised of silica glass and has a first core and first cladding surrounding the first core, and the second optical fiber is mainly comprised of silica glass and has a second core and second cladding surrounding the second core, the optical fiber line includes a first stationary section defined in the first optical fiber, a second stationary section defined in the second optical fiber, and a transition section located between the first stationary section and the second stationary section, the transition section having a mode field diameter MFD transitioning along a longitudinal direction of the optical fiber line, in the first stationary section, the first optical fiber has an effective area A eff of 90 μm 2 or less at a wavelength of 1550 nm and a mode field diameter MFD having a fluctuation range of less than 1.0 um along the longitudinal direction, an inner region of the first cladding, in the first cladding, adjacent to the first core contains fluorine of 4000 to 15000 ppm, in the second stationary section, the second optical fiber has an effective area A eff of 100 to 200 μm 2 at the wavelength of 1550 nm and a mode field diameter MFD having a fluctuation range of less than 1.0 um along the longitudinal direction, in the transition section, a connection loss expressed in decibels of a fundamental mode is equal to or less than 55% of an ideal butt loss α expressed in decibels at the wavelength of 1550 nm, wherein α = − 10 log 10 2 W 12 W 12 2 + 1 2 and W 12 denotes a value obtained by dividing the MFD of the second stationary section of the second optical fiber by the MFD of the first stationary section of the first optical fiber, and the second optical fiber has a refractive index profile of a ring core type in that the second core comprises a center core and a ring core surrounding the center core and being higher in refractive index than the center core, and the second cladding comprises an inner region and an outer region, the outer region surrounding the inner region and being higher in refractive index than the inner region, the inner region surrounding the ring core and being lower in refractive index than the ring core, so that a mode field diameter MFD of the second optical fiber becomes smaller than a case where the second optical fiber has a refractive index profile of a step type. a first optical fiber; and a second optical fiber having an end connected to an end of the first optical fiber by fusion-splicing, wherein the first optical fiber is mainly comprised of silica glass and has a first core and first cladding surrounding the first core, and the second optical fiber is mainly comprised of silica glass and has a second core and second cladding surrounding the second core, the optical fiber line includes a first stationary section defined in the first optical fiber, a second stationary section defined in the second optical fiber, and a transition section located between the first stationary section and the second stationary section, the transition section having a mode field diameter MFD transitioning along a longitudinal direction of the optical fiber line, in the first stationary section, the first optical fiber has an effective area A eff of 90 μm 2 or less at a wavelength of 1550 nm and a mode field diameter MFD having a fluctuation range of less than 1.0 um along the longitudinal direction, an inner region of the first cladding, in the first cladding, adjacent to the first core contains fluorine of 4000 to 15000 ppm, in the second stationary section, the second optical fiber has an effective area A eff of 100 to 200 μm 2 at the wavelength of 1550 nm and a mode field diameter MFD having a fluctuation range of less than 1.0 um along the longitudinal direction, in the transition section, a connection loss expressed in decibels of a fundamental mode is equal to or less than 55% of an ideal butt loss α expressed in decibels at the wavelength of 1550 nm, wherein α = − 10 log 10 2 W 12 W 12 2 + 1 2 and W 12 denotes a value obtained by dividing the MFD of the second stationary section of the second optical fiber by the MFD of the first stationary section of the first optical fiber, and the second optical fiber has a refractive index profile of a ring core type in that the second core comprises a center core and a ring core surrounding the center core and being higher in refractive index than the center core, and the second cladding comprises an inner region and an outer region, the outer region surrounding the inner region and being higher in refractive index than the inner region, the inner region surrounding the ring core and being lower in refractive index than the ring core, so that a mode field diameter MFD of the second optical fiber becomes smaller than a case where the second optical fiber has a refractive index profile of a step type.