PURPOSE: To obtain a thin film having excellent optical characteristics, mainly in visible light region by subjecting a single crystal thin film of lithium niobate containing boron to liquid epitaxial growth on a heterogeneous single crystal substrate different from a lithium niobate single crystal in the basic material using a Li2O3-B2O3-Nb2O5-based molten body flux.

CONSTITUTION: A heterogeneous substrate consisting of a heterogeneous material different from lithium niobate single crystal in the basic composition is brought into contact with Li2O3-B2O3-Nb2O5-based molten body and a single crystal thin film of lithium niobate containing boron is formed in a lattice-matching state on the heterogeneous substrate by heteroepitaxial growth. The heterogeneous substrate is preferably a single crystal substrate having hexagonal structure. Na2O and/or MgO is preferably contained in the molten body flux.