To shorten the time necessary for removing a carbon fluoride (CF) film adherent to the inside of a vacuum vessel, and to protect the surface of the mounting table during the cleaning. After a CF film is formed in, for example, a plasma treatment device, O2 gas as a cleaning gas is introduced into a vacuum vessel (2) to remove the CF film adherent to the inside of the vessel (2). In the cleaning, the O2 gas is converted to a plasma to yield active oxygen species, which physically or chemically cleave C-C bonds and C-F bonds on the surface of the CF film. The O2 gas permeates into the CF film through the cleaved sites and is then reacted with carbon of the CF film to yield CO2, which flies out. On the other hand, fluorine flies out as F2. Thus, the CF film is removed.