A method for producing a high-quality barium titanyl salt includes using, as the fluids to be treated, at least two kinds of fluids, namely, a barium titanium mixed solution that is obtained by dissolving both a barium compound and a titanium compound in a solvent, a compound solution that is obtained by dissolving, in a solvent, a compound capable of deposing the barium and titanium contained in the barium-titanium mixed solution into a barium titanyl salt, and if necessary, one or more other fluids; and mixing these fluids together in a thin film fluid formed at least between two treating surfaces and to form a barium titanyl salt. The treating surfaces are so arranged as to face each other in an approachable/separable state with one of the treating surfaces and being capable of turning relatively to the other.