Abstract: The present invention provides a composite oxide thin film which is characterized in that said thin film is formed, by energizing a work electrode and an opposite electrode immersed in a solution containing reactive components, through the reaction between said reactive components in the solution and said work electrode. More particularly, the present invention provides a composite oxide thin film formed through an electric-chemical reaction under water thermal conditions. According to the present invention, improvement of crystallinity is promoted by the use of water thermal conditions as compared with the conventional thin film forming methods, and it is possible to obtain a uniform composite oxide thin film having an excellent crystallinity directly at a relatively low temperature. A large-area thin film can thus easily be manufactured.