An FBG encoder as an optical waveguide device includes plural unit FBGs respectively including a periodic refractive index modulation structure in a longitudinal direction of an optical fiber and aligned in the longitudinal direction of the optical fiber; and a phase shift section formed between any two adjacent unit FBGs among the plural unit FBGs, thereby producing a predetermined phase difference between the periodic refractive index modulation structures of two adjacent unit FBGs, wherein the unit FBGs respectively have a structure wherein a period of the periodic refractive index modulation of the plural unit diffraction grating sections gradually increases or decreases in the longitudinal direction of the optical fiber or a structure wherein amplitude of the periodic refractive index modulation is varied relative to the longitudinal direction of the optical fiber so that an envelope of the periodic refractive index modulation is a predetermined window function.