Method for producing a large size mirror from a thin film, involving (a) the attachment of a thin film to a suction chamber, the edge of which has the required geometry of the mirror, and (b) the production of a controllable partial vacuum in the suction chamber so that the thin film takes on the required geometry of the mirror. The method is characterized in that the region of the mirror having the required geometry is enlarged by preshaping the thin film, prior to the attachment thereof to the suction chamber, by controlled tensioning in a closed pressurization chamber comprising two circular discs with a common axis and a thin film which is attached to the edges of the discs and the lateral sides of which are conjoined so as to form a conical surface, and by additionally tensioning the thin film by means of a mechanism for moving the discs apart along the common axis thereof. In order to facilitate and accelerate the process of forming a mirror from a thin film, the pressurization chamber together with the thin film attached thereto is placed in a thermal box in which the film is uniformly heated to an optimum temperature for deforming the film under increased pressure in the pressurization chamber.