PURPOSE: To obtain the device for optical memory free from light scattering on the surface and high in storage capacity by applying the ceadding layer having a lower refractive index than the core part of the core part consisting of a lithium niobate single crystal doped with Fe.

CONSTITUTION: The lithium niobate single crystal fiber 23 doped with Fe is produced, and separately, a diffusion bath 24 consisting of the sintered body of the mixture of niobium oxide and lithium oxide is formed. Then, the core part is formed implanting the single crystal fiber into the diffusion bath. At least one ion selected from the group consisting of alkali metal and alkaline earth metal is diffused internally into the surface part of the single crystal fiber 23 while sintering the obtained core part embedded in the diffusion bath at above 800°C, and the ceadding layer having a lower refractive index than the core part is formed.