Method of producing lithium niobate single crystal substrate

A method of producing a lithium niobate single crystal substrate, comprising the steps of:

• burying a lithium niobate single crystal grown by the Czochralski process, wherein the lithium niobate single crystal has a Fe concentration of 50 mass ppm or more and 2000 mass ppm or less in the single crystal and is in a form of an ingot, in an Al powder or a mixed powder of Al and Al2O3, and then
• heat-treating the buried lithium niobate single crystal at a temperature of 450° C. or more and less than 660° C., which is a melting point of aluminum, to produce the lithium niobate single crystal substrate, which has a volume resistivity controlled to be within a range of 1×108 Ω·cm or more to 2×1012 Ω·cm or less,
• wherein the lithium niobate single crystal in the form of an ingot to be heat-treated is prepared by performing a cylindrical abrading process on a lithium niobate single crystal ingot after growth of the single crystal, and
• wherein a surface roughness of the lithium niobate single crystal ingot after the cylindrical abrading process is 0.2 urn or more and 2 μm or less in arithmetic average roughness Ra.