To make it possible to control the ratio of grating writing portion (refractive index modulation portions) by changing the positional relation during grating writing, thereby changing the refractive index modulation points.

When the positional relation between a phase mask 6 and a photosensitive fiber 10 is provided with an inclination, since the photosensitive fiber 10 crosses the direction diagonal to the direction where the intensity of diffracted light 8 is intensified, the period of the portions, where the refractive index changes, is made wider than when the fiber is installed in parallel therewith (1/2 of the period (Λ) of the phase mask). The spread of the period is equal with respect to the extension direction of the photosensitive fiber 10. When the end on the opposite side is fixed and the phase mask 6 and a fiber holder are provided with similar inclination, the offset symmetrical with the previous case occurs. Only the ratio of the refractive index modulation portions (refractive index change parts 12) is thus changed without changing the period of the gratings.