PURPOSE: To eliminate the need of installations for storage and feed of fresh water and to decrease the thickness of the cylindrical body of an aerating tank consisting of a cylindrical body having no bottom plate by filling a stabilizer into a deep vertical hole excavated under the ground, then sinking the aerating tank into the vertical hole then fixing the tank.

CONSTITUTION: After a vertical hole 1 is excavated, a cylindrical body 2 of an aeration tank having no bottom plate is sunk into the hole 1 filled with stabilizing liquid A; thereafter, mortar D is poured into the deepest part of the hole 1 up to a required height, whereby the bottom bed of the body 2 is formed. After mortar C is filled between the hole 1 and the body 2 in order to support the body 2 and to perevent the falling thereof, mixed powder of cement and sand is poured therebetween to allow the filled stabilizing liquid and mud water to harden. The stabilizing liquid and mud water remaining in the body 2 are pumped up onto the ground and are subjected to a purification treatment.