PURPOSE: To obtain easily the titled cover of a uniform thickness with high reproducibility by adhering a frame to a thin nitrocellulose film formed on a smooth substrate and by immersing the film in water to strip the substrate.

CONSTITUTION: A smooth substrate such as a silicon substrate having ≥λ/4 smoothness is coated with a soln. contg. 3W15wt% nitrocellulose having 10.7W 12.2% nitrogen content by means of a spin coater under conditions of 25W30°C casting temp., 5W10cc extent of casting and 500W3,000r.p.m. number of revolutions of the spinner to form a thin nitrocellulose film of 0.5W3μm thickness. A frame is adhered to the film, and the film is immersed in water for ≥15min to strip the substrate. The film is then taken out of the water and dried. The thin nitrocellulose film can be stretched tight on the frame.