Abstract: In a method of manufacturing a semiconductor device, in an EUV scanner, an EUV lithography operation using an EUV mask is performed on a photo resist layer formed over a semiconductor substrate. After the EUV lithography operation, the EUV mask is unloaded from a mask stage of the EUV scanner. The EUV mask is placed under a reduced pressure below an atmospheric pressure. The EUV mask is heated under the reduced pressure at a first temperature in a range from 100° C. to 350 C°. After the heating, the EUV mask is stored in a mask stocker.