To clean an optical substrate of fluorite or the like directly before film deposition without damaging the optical substrate.

The method comprises: a stage where an optical substrate heated by firing vapor deposition is carried into a film deposition tank without substantially being contacted with the air; a stage where oxygen plasma is generated inside the film deposition tank, and the optical substrate is cleaned; a stage where gaseous oxygen inside the film deposition tank is scavenged; and a stage where an optical thin film is deposited on the optical substrate inside the film deposition tank.