PURPOSE: To prevent the lowering of polishing accuracy and allow the sticking azimuth of a fixture to be freely set by using fragile material with the same Knoop hardness as the Knoop hardness in the crystal orientation of the polished surface of lithium niobate, as the fixture.

CONSTITUTION: At the time of polishing lithium niobate, fragile material with the same Knoop hardness as the Knoop hardness in the crystal orientation of the polished surface of lithium niobate is used as a fixture. The plate-like lithium niobate 1, for instance, and the fixture 2 formed into the same shape are stuck to a disc-like sticking jig 3 and screwed with the lower shaft 9 of a polishing device. A bowl 7 of bottomed cylinder shape is mounted to the lower shaft 9 in such a way as to surround the sticking jig 3, and polishing material 8 is filled in the bowl 7. A polisher 4 is further disposed opposedly, and polishing load is imparted to the polisher 4 by a cylinder 6 through an ornamental hairpin shaped member 5. The hardness of both polished surface of the lithium niobate 1 and fixture 2 is thereby maintained uniformly.