Vacuum degassing method for molten glass

The present invention provides a reduced pressure to the reduced pressure which can provide the optimum viscosity plurality of different non-alkali glass defoaming conditions of non-alkali glass degassing method. Vacuum molten glass degassing method is carried out the vacuum degassing method for molten glass through the molten glass is maintained at a reduced pressure under reduced pressure defoaming flow within the vessel, characterized in that the molten glass is no alkali glass, the SO3 concentration by vacuum viscosity η of the molten glass during degassing vessel (Pa · s) and the molten glass, [SO3] (ppm) satisfy the embodiment of the vacuum degassing condition formula (1). 18.2 + 1003 / η-1.05 × [SO3] ≧ 8 (1).