PURPOSE: To provide a method of forming a thin film tapered structure, which is capable of obtaining an efficient waveguide optical coupling when the method is applied to the formation of the tapered structure of an optical waveguide element, is capable of obtaining also a positioning accuracy of the same degree as that in a photolithography method and is capable of forming also a tapered structure having an extremely large ratio of taper.

CONSTITUTION: A method of forming a thin film tapered structure has a process, in which such three layers of first, second and third thin film layers, 2, 3 and 4 as they are etched with the same etching solution and their etching rates to the etching solution are increased as they are upper layers are laminated in order on a substrate 1 in the order of these layers 2, 3 and 4, a refractive index of the layer 3 is made larger than that of the layer 2, a mask 5 is formed on the layer 4 and the layers 4, 3 and 2 are selectively etched with an etching solution using the mask 5. In this method, the thin film tapered structure is formed into a constitution wherein the end point of the etching is provided in the middle of the layer 2.