An optical fiber (1; 2) comprising: a core mainly comprised of silica glass and extending along a fiber axis (AX); and a cladding mainly comprised of silica glass, the cladding surrounding an outer peripheral surface of the core and having a refractive index lower than a refractive index of the core , wherein an average value n1_ave of the refractive index of the core , a minimum value nc_min of the refractive index of the cladding , and a refractive index n0 of pure silica glass satisfy relationships of: n1_ave > nc_min ; and nc_min < n 0 , the cladding contains fluorine, and a fluorine concentration in the cladding is adjusted to be minimum in an outermost portion of the cladding ; wherein the outermost portion is an annular region including the outer peripheral surface of the cladding and is positioned outside the position where the distance along the radial direction from the inner peripheral surface of the cladding is 80% of the width of the cladding ; and characterized in that a fluorine concentration distribution XF(r) is determined by the fluorine concentration in the cladding and a distance r along a radial direction of the optical fiber (1; 2) from the fiber axis (AX), in a cross section of the optical fiber (1; 2) perpendicular to the fiber axis (AX), and a differential coefficient XF'(r) of the fluorine concentration distribution XF(r) at the distance r is negative, and an absolute value of the differential coefficient XF'(r) is 30 ppm/μm or more and 600 ppm/μm or less in any small section, the small section having a width of 1 μm defined along the radial direction, in an outer region of the cladding , the outer region being defined as an annular region from a position where a distance along the radial direction from an inner peripheral surface of the cladding is 50% of a width of the cladding to a position where a distance along the radial direction from the inner peripheral surface is 80% of the width of the cladding . a core mainly comprised of silica glass and extending along a fiber axis (AX); and a cladding mainly comprised of silica glass, the cladding surrounding an outer peripheral surface of the core and having a refractive index lower than a refractive index of the core , wherein an average value n1_ave of the refractive index of the core , a minimum value nc_min of the refractive index of the cladding , and a refractive index n0 of pure silica glass satisfy relationships of: n1_ave > nc_min ; and nc_min < n 0 , the cladding contains fluorine, and a fluorine concentration in the cladding is adjusted to be minimum in an outermost portion of the cladding ; wherein the outermost portion is an annular region including the outer peripheral surface of the cladding and is positioned outside the position where the distance along the radial direction from the inner peripheral surface of the cladding is 80% of the width of the cladding ; and characterized in that a fluorine concentration distribution XF(r) is determined by the fluorine concentration in the cladding and a distance r along a radial direction of the optical fiber (1; 2) from the fiber axis (AX), in a cross section of the optical fiber (1; 2) perpendicular to the fiber axis (AX), and a differential coefficient XF'(r) of the fluorine concentration distribution XF(r) at the distance r is negative, and an absolute value of the differential coefficient XF'(r) is 30 ppm/μm or more and 600 ppm/μm or less in any small section, the small section having a width of 1 μm defined along the radial direction, in an outer region of the cladding , the outer region being defined as an annular region from a position where a distance along the radial direction from an inner peripheral surface of the cladding is 50% of a width of the cladding to a position where a distance along the radial direction from the inner peripheral surface is 80% of the width of the cladding . <EOS>