An object of the present invention is to provide a method and an apparatus for producing a large sized filmed substrate such as a reflection preventive filter using a relatively small production facility. In a method for producing a filmed substrate according to the present invention, the thickness of a monitor film formed on a film forming monitor is measured by using the film forming monitor disposed outside of a film forming area or a movable film forming monitor and a film forming process is controlled based on a film thickness so measured to thereby form a film even on a film forming objective substrate having a size equal to the entirety of the film forming area. In addition, the method of the present invention can improve the degree of freedom in which the film forming objective substrate is disposed within the film forming area, whereby the productivity during the production process can also be improved.