To prevent the lowering of alkali removing efficiency even if the operation of an electrodialyser is resumed after once stopped, in a method for manufacturing an alkali removed water glass aqueous solution suitably usable as a foundation improving agent by supplying a water glass aqueous solution to the desalting chamber of the electrodialyser to apply alkali removing treatment thereto.

The water glass aqueous solution is supplied to the desalting chamber of the electrodialyser to be electrodialyzed to manufacture the alkali removed water glass aqueous solution wherein the concentration of SiO2 is 1 wt.% or more, a molar ratio of SiO2/X2O, (X is an alkali metal atom) is 6 or more and pH is 7-12. In this manufacturing method, the desalting chamber is drained and washed within 120 min immediately after the completion of electrodialysis not only to set the electroconductivity of the aqueous solution in the desalting chamber to 3,000 μS/cm or less but also to set the concentration of SiO2 in the aqueous solution to 2 g/l or less.