A method of producing a lithium-tantalate crystal, wherein at least a first material containing lithium tantalate, lithium niobate or hydrogen storage alloy storing hydrogen that is subjected to a heat treatment at a temperature of T1' that is Curie temperature or higher in a reducing atmosphere is superposed on a single-polarized lithium-tantalate crystal, and then the crystal is subjected to a heat treatment at a temperature of T2' that is lower than Curie temperature in a reducing atmosphere, thereby an electric conductivity of the single-polarized lithium-tantalate crystal is increased. There can be provided a method of producing a lithium-tantalate crystal wherein the surface charge generated by applying a temperature change to the lithium-tantalate crystal can be decayed quickly without accumulating by increasing the electric conductivity, and an effective piezoelectric property is exhibited by maintaining the single polarized structure.