A fiber laser device comprising: a multimode amplification optical fiber having a core doped with an active element that emits light in a pumped state; a first fiber Bragg grating, FBG, formed on a core of a multimode optical fiber disposed on a first side of the amplification optical fiber, the first FBG being formed of a plurality of high-refractive index portions at a predetermined interval, the high-refractive index portion having a refractive index higher than a refractive index of the core on which the first FBG is formed, the first FBG reflecting at least a part of light emitted from the active element; and a second fiber Bragg grating, FBG, formed on a core of a multimode optical fiber disposed on a second side of the amplification optical fiber, the second FBG being formed of a plurality of high-refractive index portions at a predetermined interval, the high-refractive index portion having a refractive index higher than a refractive index of the core on which the second FBG is formed, the second FBG reflecting the light reflected off the first FBG at a reflectance lower than a reflectance of the first FBG, wherein: a wavelength of a fundamental-mode light beam reflected off the first FBG and a wavelength of a fundamental-mode light beam reflected off the second FBG are matched with each other; and at the high-refractive index portion of at least one of the first FBG and the second FBG, a refractive index of the core is ununiform on a cross section perpendicular to a longitudinal direction, such that wavelengths of higher-mode light beams reflected off the first FBG and wavelengths of higher-mode light beams reflected off the second FBG are unmatched with each other. a multimode amplification optical fiber having a core doped with an active element that emits light in a pumped state; a first fiber Bragg grating, FBG, formed on a core of a multimode optical fiber disposed on a first side of the amplification optical fiber, the first FBG being formed of a plurality of high-refractive index portions at a predetermined interval, the high-refractive index portion having a refractive index higher than a refractive index of the core on which the first FBG is formed, the first FBG reflecting at least a part of light emitted from the active element; and a second fiber Bragg grating, FBG, formed on a core of a multimode optical fiber disposed on a second side of the amplification optical fiber, the second FBG being formed of a plurality of high-refractive index portions at a predetermined interval, the high-refractive index portion having a refractive index higher than a refractive index of the core on which the second FBG is formed, the second FBG reflecting the light reflected off the first FBG at a reflectance lower than a reflectance of the first FBG, wherein: a wavelength of a fundamental-mode light beam reflected off the first FBG and a wavelength of a fundamental-mode light beam reflected off the second FBG are matched with each other; and at the high-refractive index portion of at least one of the first FBG and the second FBG, a refractive index of the core is ununiform on a cross section perpendicular to a longitudinal direction, such that wavelengths of higher-mode light beams reflected off the first FBG and wavelengths of higher-mode light beams reflected off the second FBG are unmatched with each other.