PURPOSE: To provide an inexpensive and long-lasting method that has an immediate effect for various kinds of bulking which are troublesome in a drainage treatment system having no effective solution, through activated sludge.

CONSTITUTION: At least one kind of calcium carbonate, calcium silicate and calcium phosphate is allowed to coexist at the quantity equivalent to 5.0-400wt.% of sludge for an activated sludge system in which bulking has been caused. The effect of additional inorganic powder is raised by adding floc forming bacteria separated from various activated sludge to the activated sludge system at the quantity of 5.0-100wt.% of the additional inorganic powder. Further the particle diameter of the used inorganic powder is preferably regulated to ≤0.8mm.