PURPOSE: An alkaline injection solution obtained by combining a nonalkaline water-glass solution with an alkaline solution is used together with a nonalkaline water glass injection solution and a salt is added to the former solution to permit easy regulation of gelation time.

CONSTITUTION: Injection into the ground of an alkaline solution obtained by combining a nonalkaline water-glass solution with an alkaline solution is combined with the injection of a nonalkaline water-glass solution and the former alkaline solution is allowed to contain a salt by adding it to one of the two components, thus effecting ground improvement. As the salt, is preferably cited an alkali metal salt, alkaline earth metal salt or aluminum salt and its content is preferably 0.2W7wt% based on the nonalkaline water-glass solution.

EFFECT: The gelation is gradually accelerated, the gel strength is increased, further, the gelation is rapidly accelerated in the alkaline zone to give a wide range of instant gelation zone.