To obtain electrooptic thin film elements each having a high electrooptic constant.

In the electrooptic thin film element, a lithium niobate single crystal thin film, characterized in that the lattice constant in an inplane direction is the same as that of a Z-cut electrooptic single crystal substrate and the lattice constant in a direction perpendicular to the surface is larger than that of the single crystal substrate, is formed on the single crystal substrate. Or, in the electrooptic thin film element, a lithium niobate single crystal thin film, characterized in that the lattice constant in an inplane direction is the same as that of an X-cut electrooptic single crystal substrate and the lattice constant in a direction perpendicular to the surface is smaller than that of the single crystal substrate, is formed on the single crystal substrate. It is preferable that the lattice strain between the Z-cut substrate and the lithium niobate single crystal thin film is ≥+0.1%, and the lattice strain between the X-cut substrate and the lithium niobate single crystal thin film is ≤-0.1%.