A storage container for containing a photomask-forming synthetic quartz glass substrate comprises inner walls facing the front and back surfaces of the glass substrate, and reservoirs provided on the container inner walls for receiving an absorber for components outgassing from the container. A ratio A/B is in the range of 1.0-120 m2/cm2 provided that A is the product (m2) of the total weight (g) of the absorber multiplied by the BET specific surface area (m2/g) of the absorber and B is the total area (cm2) of the front and back surfaces of the glass substrate.