Lithium niobate single crystal substrate and method of producing the same

A method of producing a lithium niobate single crystal substrate by using a lithium niobate single crystal grown by the Czochralski process, wherein

• a lithium niobate single crystal having a Fe concentration of more than 1000 mass ppm and 2000 mass ppm or less in the single crystal and processed into a form of a substrate is buried in an Al powder or a mixed powder of Al and Al2O3, and heat-treated at a temperature of 550° C. or more and 600° C. or less, to produce a lithium niobate single crystal substrate having a volume resistivity controlled to be within a range of 1×108 Ω·cm or more to 1×1010 Ω·cm or less, and
• the variation (σ/Ave) in the volume resistivity in the plane of the lithium niobate single crystal substrate is less than 3%, wherein
• Ave is an average value of volume resistivities measured at one point in a center portion and four points on an outer peripheral portion, five points in a surface in total, of the lithium niobate single crystal substrate, and σ is a standard deviation of the volume resistivities, and the volume resistivities are values measured by a three-terminal method according to JIS K-6911.