PURPOSE: To minimize the turblence of a liquid stream after fine air bubbles are generated in a flotation tank, by providing an empty chamber, of which the bottom wall is isolated from the bottom of a flotation concentrating tank and side wall is protruded from the water level in the flotation concentrating tank, to the central part of the flotation concentrating tank.

CONSTITUTION: A liquid mixture downwardly injected into a tank from an outlet 6 is buffered by a barrier plate 11 to receive the change in the flow direcrion thereof and flows radially and equally toward four peripheries between an air bottom wall 2b and the barrier plate 11. The conc. sludge of the froth layer on a water level is scraped up by a scraper 12 and sent into the outflow culvert of the tank periphery to be discharged out of the tank. Separated water, which is separated from sludge in a separation zone, is equally collected by water cellecting pipes 14 arranged in a concentrical ring shaped form within the tank and taken out from said pipes. Falling sludge is precipitated to the tank bottom 1b of which the inclination angle is relatively large and gathered to the central part to be discharged out of the tank therefrom.