A slanted fiber grating comprising: an optical fiber comprised of silica-based glass, the optical fiber including a core extending along a fiber axis direction, a first cladding surrounding the core in a cross section perpendicular to the fiber axis direction, the first cladding having at least a portion containing a photosensitive material and having a refractive index lower than a refractive index of the core, the photosensitive material increasing a refractive index of the portion by irradiation with light with a specific wavelength, and a second cladding surrounding the first cladding in the cross section, the second cladding having a refractive index that is lower than the refractive index of the core and higher than the refractive index of the first cladding, wherein a specific section located between two different points arranged along the fiber axis direction in the optical fiber is configured with a first region having a first mode field diameter at a wavelength of 1.55 μm and including a slanted Bragg grating having an iso-refractive index surface tilted to the cross section, the slanted Bragg grating being provided in a region corresponding to the first cladding, a pair of second regions arranged along the fiber axis direction to sandwich the first region, and a pair of third regions arranged along the fiber axis direction to sandwich both the first region and the pair of second regions, the pair of third regions having a second mode field diameter smaller than the first mode field diameter at the wavelength of 1.55 μm, characterized in that: a length of the first region defined along the fiber axis direction is from 10 mm to 100 mm, and the first region includes an MFD adjustment Bragg grating disposed at a position different from the slanted Bragg grating along the fiber axis direction, the MFD adjustment Bragg grating has an iso-refractive index surface perpendicular to the fiber axis direction and Bragg-reflects light in a wavelength band different from a loss wavelength band of the slanted Bragg grating. an optical fiber comprised of silica-based glass, the optical fiber including a core extending along a fiber axis direction, a first cladding surrounding the core in a cross section perpendicular to the fiber axis direction, the first cladding having at least a portion containing a photosensitive material and having a refractive index lower than a refractive index of the core, the photosensitive material increasing a refractive index of the portion by irradiation with light with a specific wavelength, and a second cladding surrounding the first cladding in the cross section, the second cladding having a refractive index that is lower than the refractive index of the core and higher than the refractive index of the first cladding, wherein a specific section located between two different points arranged along the fiber axis direction in the optical fiber is configured with a first region having a first mode field diameter at a wavelength of 1.55 μm and including a slanted Bragg grating having an iso-refractive index surface tilted to the cross section, the slanted Bragg grating being provided in a region corresponding to the first cladding, a pair of second regions arranged along the fiber axis direction to sandwich the first region, and a pair of third regions arranged along the fiber axis direction to sandwich both the first region and the pair of second regions, the pair of third regions having a second mode field diameter smaller than the first mode field diameter at the wavelength of 1.55 μm, a length of the first region defined along the fiber axis direction is from 10 mm to 100 mm, and the first region includes an MFD adjustment Bragg grating disposed at a position different from the slanted Bragg grating along the fiber axis direction, the MFD adjustment Bragg grating has an iso-refractive index surface perpendicular to the fiber axis direction and Bragg-reflects light in a wavelength band different from a loss wavelength band of the slanted Bragg grating.