PURPOSE: To mix a ground improving agent according to depths by a method in which at least quick lime is confined in a bag breakable at low temperature and fine holes are formed in the bag to form an improving agent bag, and the bags are supplied and mixed with the ground of the same depth as the stirring blade.

CONSTITUTION: A ground improving agent bag 1 made of a synthetic resin meltable at comparatively low temperature, having fine holes 2 through which water can pass, is filled with quick lime with or without cement separately. A ground improving agent-packed tube 10 vertically movable along the stirring blade 8 of a general ground stabilizing device is provided for the ground stabilizing device. The tube 10 and the blade 8 are penetrated into the ground to be stabilized to mix soil with improving agents 3 and 4 by the blade 8 by breaking the bags 1 by reaction of water with the improving agent in the bag 1 discharged from the tube 10. Mixing of soil with improving agent can thus be attained according to depths.