A method for fabricating fiber Bragg gratings including: scanning a photosensitive optical fiber with ultraviolet laser light in a longitudinal direction of the optical fiber by means of a phase mask method, thereby forming periodic refractive index modulation structure in a core of the optical fiber in the longitudinal direction; and instantaneously moving a phase mask used in the phase mask method by a predetermined distance in the longitudinal direction, thereby forming a phase shift portion in the periodic refractive index modulation structure formed in the core of the optical fiber, when a radiation position of the ultraviolet laser light reaches a predetermined position, in the middle of the scanning step using the ultraviolet laser light.