PURPOSE: To easily manufacture thin Al films each holding its mirror surface and having an arbitrary thickness by suspending the formation of an Al film several times, forming thin films of a metal of a different kind during the suspension, and laminating Al films.

CONSTITUTION: A thin Al film 21 is formed on a glass substrate 11 polished to an optical flat surface by a vacuum deposition method, and a very thin Cr film 31, for example, is formed on the film 21 in the same vacuum. Al films 22, 23 and Cr films 32, 33 having the same thickness are alternately formed in the same way, and finally an Al film 24 is formed. Since the Al films are separated from one anothe by the Cr films, the grains of the Al films are not grown, so the Al films maintain their mirror surfaces.