Abstract: In an apparatus and a method of the invention, a large amount of fluid is processed in a short time, and it is possible to use even in a square tank without losing certain and desirable stirring. The apparatus includes a fluid tank for accommodating fluid therein, and at least one agitating device situated in the tank. The tank includes a bottom having a slope to allow the fluid to flow from one side of the bottom to the other side thereof, and a plurality of discharge ports disposed near the other side of the bottom. The agitating device includes a rotation axis suspended from an upper portion of the tank, a set of stirring blades attached to a lower part of the rotation axis and extending radially outwardly from the rotation axis, and a rotating device engaging the rotation axis. When the rotating device is actuated, the stirring blades can stir the fluid in the tank.