An optical fiber comprising: an incident end adapted to receive light incident on the incident end; an emitting end adapted to emit light received by the incident end, an aperture which is provided in a core, located near the emitting end, the aperture being formed by irradiating the core of an optical fiber of a workpiece in which the aperture is not formed yet with an ultrashort pulsed laser beam having pulse widths of 10 -15 seconds to 10 -11 seconds, and a clad covering an outside of the core, wherein the aperture has a disc shape, includes an opening in a center thereof so as to surround an axis line of the core, and is formed by a light scattering region, the light scattering region being formed by inducing a damage change in part of the core through the irradiation of the ultrashort pulsed laser beam in a position, which is retreated from the emitting end along the axis line by a distance, the optical fibre being characterised in that the light scattering region is retreated from the emitting end along the axis line by said distance being within a range, where at least part of the light passing through a portion of said core where said opening of said light scattering region is provided, reaches the emitting end while being reflected at the interface between the core and the clad on an emitting end side of the light scattering region such that a beam spot of the light emitting from the emitting end in an observation plane includes a small beam spot with a diameter b and having a high radiation intensity, which small beam spot is the region irradiated with said at least part of the light passing through a portion of said core where said opening of said light scattering region is provided, reaches the emitting end while being reflected at the interface between the core and the clad on an emitting end side of the light scattering region, a large beam spot with a diameter c and having a low radiation intensity, which large beam spot is the region irradiated with light, which is not incident to the clad but emits from the emitting end in the light passing through the light scattering region to weaken the radiation intensity, and an intermediate beam spot with a diameter d and having the diameter and radiation intensity intermediate between the small and large beam spots (B, C), which intermediate beam spot is the region irradiated with the light, which emits from the emitting end while neither passing through the light scattering region 5a nor being reflected at the interface between the core and the clad, wherein the relationship among the diameters of the respective beam spots satisfies a condition of b < d ≤ c. an incident end adapted to receive light incident on the incident end; an emitting end adapted to emit light received by the incident end, an aperture which is provided in a core, located near the emitting end, the aperture being formed by irradiating the core of an optical fiber of a workpiece in which the aperture is not formed yet with an ultrashort pulsed laser beam having pulse widths of 10 -15 seconds to 10 -11 seconds, and a clad covering an outside of the core, wherein the aperture has a disc shape, includes an opening in a center thereof so as to surround an axis line of the core, and is formed by a light scattering region, the light scattering region being formed by inducing a damage change in part of the core through the irradiation of the ultrashort pulsed laser beam in a position, which is retreated from the emitting end along the axis line by a distance, the optical fibre being characterised in that the light scattering region is retreated from the emitting end along the axis line by said distance being within a range, where at least part of the light passing through a portion of said core where said opening of said light scattering region is provided, reaches the emitting end while being reflected at the interface between the core and the clad on an emitting end side of the light scattering region such that a beam spot of the light emitting from the emitting end in an observation plane includes a small beam spot with a diameter b and having a high radiation intensity, which small beam spot is the region irradiated with said at least part of the light passing through a portion of said core where said opening of said light scattering region is provided, reaches the emitting end while being reflected at the interface between the core and the clad on an emitting end side of the light scattering region, a large beam spot with a diameter c and having a low radiation intensity, which large beam spot is the region irradiated with light, which is not incident to the clad but emits from the emitting end in the light passing through the light scattering region to weaken the radiation intensity, and an intermediate beam spot with a diameter d and having the diameter and radiation intensity intermediate between the small and large beam spots (B, C), which intermediate beam spot is the region irradiated with the light, which emits from the emitting end while neither passing through the light scattering region 5a nor being reflected at the interface between the core and the clad, wherein the relationship among the diameters of the respective beam spots satisfies a condition of b < d ≤ c.