PURPOSE: To permit supporting of even a thin mirror without making high- accuracy pressure control by providing bellows between the respective circumferential edges of a mirror cell and mirror to form a closed space between both, sealing gas in this space and supporting the mirror by the pressure of said gas.

CONSTITUTION: The closed space is formed between the mirror cell 2 and the mirror 1 by providing 1st and 2nd bellows 9, 10 therebetween and the mirror 1 is supported by sealing the gas into this space and, therefore, a pressure can be uniformly exerted to the mirror over the entire rear surface thereof and the thin mirror 1 can be thereby supported even if the pressure control is not executed; in addition, there is no need for a pad to support the mirror 1. Furthermore, the wall thickness of the mirror 1 can be balanced with the weight and pressure of the mirror 1 and since the vertical position of the mirror 1 is controlled by controlling the pressure of the air by a controller 27, high accuracy is not needed for an apparatus for pressure control.